THE 
EFFICACY OF FILTERS AND OTHER 
MEANS EMPLOYED TO PURIFY 
DRINKING-WATER. 
A Bacteriological Study. 
BY 
CHARLES G. D., 
OF NEW kUKK." 
FROM 
THE MEDICAL NEWS, 
April 20 and 27, 1889. [Reprinted from The Medical News, April 20 and 27,1889.] 
THE EFFICACY OF FILTERS AND OTHER MEANS 
EMPLOYED TO PURIFY DRINKING-WATER. 
A Bacteriological Study.1 
By CHARLES G. CURRIER, M.D., 
OF NEW YORK. 
Water, even after having been exposed to various 
chances of contamination, is very generally drunk 
as it is received from lakes or rivers, wells or springs, 
tanks or hydrants; and, despite the most disinter- 
ested warnings, people are not ready enough to 
believe that clear, sparkling water may at times be 
the most harmful of beverages; that it can carry with 
it, and introduce into the systems of a portion at 
least of those who drink it, the immediate causes 
of various impairments of health. Polluted water 
may be agreeable to the taste and have no visible 
sediment, yet cause many deaths. Such water the 
unskilled are slow to suspect, and but for the ad- 
vance in sanitary science and the warnings conveyed 
by those familiar with hygienic studies, pure sup- 
plies of drinking-water in cities would be much more 
of a rarity than they are. 
1 Read before the Section on Public Health of the New York 
Academy of Medicine, April 5, 1889. 2 
CURRIER, 
At the Hygienic Congress in Vienna, in 1887, M. 
Brouardel stated that he knew of over sixty epi- 
demics of typhoid fever which had been induced 
by the drinking of infected water.1 From typhoid 
fever thousands of deaths occur every year; and, 
although scientific physicians are constantly urging 
these facts, it seems to be necessary that the danger 
present itself in the form of an epidemic before the 
people fully realize the immense harm which may 
result from impure drinking water. The epidemic 
which ravaged Plymouth,2 in Pennsylvania, four 
years ago, attracted great attention at the time, but 
the history of several similar “visitations” since 
then would indicate that the full value of this severe 
lesson has not been properly appreciated. 
Water may be very highly colored, and yet, if 
freshly collected near its source, show no very large 
number of bacteria. Such is the case with the best 
of the peaty “juniper” water which I have examined 
in various parts of the Dismal Swamp of Carolina 
and Virginia. Although having a large propor- 
1 Dr. A. Ollivier: Rapport au Conseil d’Hygiene Publique 
(etc.), Paris, 26 mars, 1886. A valuable monograph on typhoid 
fever by a scientific practitioner. As different parts of Paris receive 
water from rivers of very different quality, the careful health 
reports from garrisons and institutions, which vary only in the purity 
of the water supplied, are very instructive. 
From the abundant evidence as to the influence of polluted 
water in inducing typhoid, may; be cited the report of the Zurich 
Water Commission of 1885. 
2 Biggs and Breneman : N. Y. Med. Journal, vol. xli. (1885), 
May 23 and June 6. French and Shakespeare: ibid., June 13. 
I,. H. Taylor: The Medical News, vol. xlvi. (1885), May 16 
and June 20. THE EFFICACY OF FILTERS. 
3 
tion of organic matter, this water is reputed emi- 
nently wholesome, and an inquiry among a portion 
of those drinking this exclusively, appears to show 
an unusual degree of immunity from the manifesta- 
tions of diseases which might be attributed to the 
effects of improper drinking-water.1 On the other 
hand, the turbidity which is common at this season 
of the year in the streams furnishing the water supply 
of many of our cities, is almost always accompanied 
by a large number of bacteria, as in these bottles of 
water which you see here from the Mississippi and 
from some of the rivers flowing into the Atlantic. 
The melting snow and the rain, which wash into 
the streams an increased amount of organic matter, 
introduce with it other germs (perhaps pernicious), 
and bring more nutriment than usual to the micro- 
organisms already in the water, thus increasing their 
vitality and numbers. 
It is believed that, like most of the varieties of 
bacteria found in water, the still larger forms of 
animal and vegetable life, ordinarily existing in 
lakes and rivers, are not prejudicial to hea'th. It 
is through the products of the decomposition of 
1 If harmful bacteria be introduced into it, such water favors 
their increase, while they would not be so apt to multiply in a 
very pure water having much less organic matter. Despite the 
gcod reputation which this water has had. and its wholesomeness 
near Lake Drummond, one who has seen the whole length of the 
canal down to the lower end, from which the supply for vessels is 
taken, can realize that the water may undergo considerable con- 
tamination before it reaohes that part. I am sure that the present 
use (in our navy) of distilled water, kept in proper receptacles, 
is safer than the employment of the swamp water. 4 
CURRIER, 
many of these, when their life has ceased, that they 
may become harmful. 
Chemistry has long furnished a means of recog- 
nizing polluted water, even when it yielded no sedi- 
ment visible to the unaided eye, and the presence of 
considerable quantities of chlorides, nitrites, and 
ammonias, determined by the chemical tests, has 
shown that many a water had more organic impurity 
than was permissible; and, when far below the ac- 
cepted standard in this respect, such a water is very 
justly condemned. Yet it seems as though enough 
of the infective element of cholera or typhoid to 
diffuse an epidemic throughout a community can 
be added to a drinking-water and not be detectable 
by chemical tests. A perusal of the very complete 
and instructive paper by Professor J. W. Mallet (in 
the report of the National Board of Health for 1882) 
shows how widely such determinations may vary and 
how unreliable chemical standards may be. 
The known disease-producing bacteria, however, 
do not appear to increase in water1 in which there 
is not more organic matter than the proportion that 
chemists of constant and large experience regard as 
permissible in waters which they pronounce to be of 
fair quality, although these harmful bacteria, if once 
1 In my tests, when I prepared an artificial water, I added the 
germs directly from a pure culture, unlike Bolton, and like Wolfif- 
hiigeland Riedel (Arb. aus d. Gesundheitsamte, 1886, i. p. 45s), 
with whose results, however, I cannot so well agree as I can with 
those of Bolton (Ztsch. f. Hygiene, 1886, i. p. 76). I have in but 
two out of many trials, with disease-producing species, found 
typhoid bacilli to increase in sterile pure water, and then only to 
the extent of thirty per cent. THE EFFICACY OF FILTERS. 
5 
introduced there, may long retain their vitality and 
remain dangerous.1 
Since, as yet, chemistry, when called upon to aid 
in solving the question of the purity of a water, can 
at its best only approximately estimate the amount 
of organic matter present and its state of oxida- 
tion, while it fails to inform us whether infective 
matter be present or not in a given water, it has 
been hoped that a sure means of recognizing whether 
a water be in any degree infected was reached when 
Koch introduced the bacteriological test. This test, 
while very definite and conclusive under favorable 
conditions, has thus far been able to decide abso- 
lutely in only a small portion of the cases of disease 
apparently coming from the use of drinking-water. 
1 Chantemesse and Vidal (Gaz Hebdomadaire de M6d. et de 
Chir., 1887, pp. 146-150) kept typhoid bacilli alive for “months ” 
in sterilized water from the Ourcq, which is the worst water sup- 
plied to Paris, and much impurer than the Croton, in which, when 
it was sterilized, I have been able to keep typhoid bacilli alive for 
only two weeks, and usually not over eight or nine days. I have 
observed that the greater the variation of temperature it was ex- 
posed to, the shorter time did this microorganism live in water. 
It has been found by all observers that oscillations of tempe- 
rature around the freezing point were much more destructive to 
these than freezing them in water and allowing the ice to re- 
main frozen. So, I have observed several times that of two flasks 
of the same typhoid water, the one which was kept at a temperature 
varying from 370 C. to io° C., had, at the end of three days, many 
fewer than were in the flask kept between 160 C. and io° C. In 
my last test of this sort, the flask of which the temperature varied 
most had, after two days, only a few more than a third of those 
in the one kept at the evener temperature. Herasus : Zeitschrift 
fur Hygiene, i., 2, p. 193. Hochstetter: Arb. aus d. Gesund- 
heitsamte, ii., 1, p. 1. 6 
CURRIER, 
Yet in those cases where the bacteria characteristic 
of distinct disease have been detected in the sus- 
pected water, the isolation, culture, and identifica- 
tion of the disease-producing species furnish a con- 
vincing proof of the great value of the method. 
The method employed is a simple one. Usually 
one cubic centimetre of the water to be tested is 
added to ten cubic centimetres of nutrient jelly, 
made by solidifying very nutritious beef-tea by the 
incorporation of one-tenth its weight of gelatine. 
Everything being done with rigid precautions, the 
water to be tested is added to the nutrient jelly, 
which has been softened by being warmed to a little 
more than 30° Centigrade. This is then quickly 
mixed and poured upon a cooled glass plate, where 
the fluidified gelatine soon becomes quite solid. 
Then the germs in the water, being scattered evenly 
throughout the mass, exercise their functions of rap- 
idly multiplying at the room temperature; and, in 
the nutritious medium, the minute microscopic indi- 
vidual has perhaps by the third day become a dot as 
large as a pin-point, or much larger, containing 
thousands or millions of newly formed bacteria. 
These are called “ colonies,” and vary considerably 
in appearance, as will be seen by comparing the 
plates made with various waters. With a glass ruled 
into centimetre squares the number can be counted 
or estimated. If a large number of thousands be 
present, as in waters artificially prepared for tests 
and in sewage contamination of water, the only way 
to arrive at an accurate estimate is to employ in the 
eye-piece of a microscope a micrometer square, as THE EFFICACY OF FILTERS. 
7 
first used, I believe, by Dr. Edw. K. Dunham, who 
combines with it the aid of an automatic counter. 
After determining the number we isolate and iden- 
tify the species as in general bacteriological study. 
There are many features of the subject which must 
be omitted in a paper of this scope, for they may 
not interest all, while the limits of the usual time 
allowed must be respected; and as the important 
points are to be discussed by eminent experts, I will 
not linger over the still unsettled question as to the 
absolute value of mere numbers of ordinary water 
bacteria in determining the sanitary merits of a 
water.1 It is, however, admitted that the fewer the 
varieties present, the less is the likelihood of sources 
of contamination affecting the water. In examin- 
ing a considerable number of samples of water from 
various sources, I have observed that the less chance 
of contamination there was, the fewer bacteria have 
I found in the waters. Thus, fine mountain lakes 
and springs have never in my experience, unless 
under very unfavorable conditions, shown as many 
as one hundred bacteria in each c. c., while the 
same quantity of water from a river draining thickly 
peopled valleys may show more than fifty times as 
many. While it offers less striking extreme differ- 
ences than I have seen in the Hudson Valley and 
1 Gartner: Correspondenz Bl. d. allg. arzt. Ver. von Thuringen, 
1888 ; Nos. 2 and 3. 
Centralbl. f. Bakt. und Par., iii., 5, p. 161 et seq. 
Wolffhiigel: Arbeiten aus dem Gesundheitsamte, Band i., iii., 
5 Heft (1885), p. 546. 
Plagge und Proskauer, Ztschr. f. Hygiene, 1887, pp. 470 and 
486. 8 
CURRIER, 
elsewhere, the Passaic water-shed may be briefly in- 
stanced as illustrating what I have just said. I give 
it because it is the smallest one near us offering every 
phase of importance in this connection, and because 
the subject of a supply to come to New York from 
that source has been seriously considered. The best 
of the upper lakes of this system showed on a pleas- 
ant day of the past winter only fifty-seven germs 
developing in the nutrient gelatine, while the Pequan- 
nock and other tributaries into which these lakes 
flow showed, in the samples collected as nearly at 
the same time as possible, over three hundred and 
fifty germs, which number increased as the water was 
taken further down stream; while at the Passaic 
Falls, in Paterson, over a thousand were detected in 
each c. c. Water from the hydrant in Newark, at 
the same time, showed nearly four thousand germs 
of bacteria in each c. c. The Newark water supply 
is at present derived from the Passaic, some miles 
below the place where sewage from Paterson is dis- 
charged into the river. 
As in chemical, so in bacteriological water exam- 
inations, those who have had the broadest experience 
and who are most careful are best in a position to 
pronounce an opinion. Apart from any personal 
equation, there are still limitations to the entire com- 
prehensiveness of the method. I may mention that 
bacteria are recognized as present in the human 
body and in its products and changed tissues, which 
bacteria are never found to live on the gelatine or 
other water plates. Then, too, a water may be sub- 
mitted for examination when all the obvious infec- THE EFFICACY OF FILTERS. 
9 
tion has disappeared, or has become exceedingly 
diluted, and hence a most painstaking test then fails 
to reveal the presence of the characteristic micro- 
organism. The period of incubation, after the in- 
fection and before the symptoms are recognized, may 
in Asiatic cholera occupy the greater part of a week, 
and in typhoid often lasts for two weeks or more, so 
that all traces of the harmful bacteria may have left 
the water before it is suspected. By reason, also, of 
its likeness to various harmless species, and its ten- 
dency to be overwhelmed by other more rapidly 
developing forms, the “colony” of the disease-pro- 
ducing species may be hard to identify. In this 
respect typhoid is decidedly more difficult than 
cholera to recognize in ordinary water. 
For those who believe that bacteria of certain kinds 
living in water, and with this introduced into the 
alimentary canal, can in many cases induce disease, 
this test, being so precise and delicate as it is, must 
appear the best way of determining whether the fil- 
ters and other means in common use are efficient or 
otherwise in removing the dangerous elements from 
impure water passed through them—in short, whether 
they act as “ disinfectants ” of the water. 
As various other agencies are used in conjunction 
with, or as supplementary to, filtration, it is proper 
that I here indicate how far they serve to purify 
drinking-water. One of the most prevalent means 
directly resorted to for this ostensible purpose is the 
admixture of wine (usually red). Adding this to an 
equal amount of Croton water, I find that a vary- 
ing proportion of the bacteria are killed, sometimes IO 
CURRIER, 
less than half, even after the mixture has stood for 
days. The spirituous alcoholic beverages (having 
from 45 to 68 per cent, of alcohol), mixed with 
Croton hydrant water in equal parts, cannot be de- 
pended upon to kill all of the bacteria, especially 
when “ earth ” bacteriaare present, and a small per- 
centage of whiskey (less than 20 per cent.) has very 
little effect in this direction.1 In malt liquors, as 
lager beer, the prolonged boiling in the brewer’s 
kettle has destroyed the bacteria of the original 
water, even if they be of the most harmful and re- 
sistive species which we meet here. Various kinds, 
however, if present in vats, kegs, and bottles (be- 
cause of imperfect cleansing), will live in beer.2 
Almost all the artificial mineral waters that I have 
examined are made of unsterilized water. Pressure 
of carbonic acid gas, and contact with it for days, 
1 Koch (Fliigge, Chapter V., sec. 3) found that the spores of 
Bacillus anthracis were alive even after months of exposure to the 
action of absolute alcohol. 
Pasteur and Joubert (Comptes Rendus des Stances de 1'Acade- 
mic des Sciences, 9 and 16 juillet, 1877) stated long ago that the 
spores of anthrax would remain alive in absolute alcohol. 
Anthrax, however, need not be expected to occur in the water 
supplied here, and several recent experiments show that it is pos- 
sibly destroyed by other bacteria which may occur in the water. 
Anthrax spores are employed in such tests because they are con- 
sidered the most resistant of all disease-producing forms of bac- 
teria. 
2 The bacteria from this source which I have found in examin- 
ing a limited number of beers were mostly of non-liquefying kinds, 
and there were notably many iridescent “spreading” colonies. 
[Jan. 1889.] 
In these cases water from old surface wells was used to wash out 
the kegs. THE EFFICACY OF FILTERS. 
11 
reduces greatly the number of bacteria in a mineral 
water, yet the typhoid bacillus and others will live 
•or days in Seltzer water. In New York I have found 
many bacteria in the few carbonated beverages that 
I have examined. 
Of the various acids which it is occasionally safe 
to employ, chemically pure hydrochloric acid must 
be used in the strength of at least one-half of one 
per cent, to destroy all the germs in Croton water to 
which B. subtilis, B. typhoid, and other bacteria have 
been added; and may fail- to sterilize the water 
even after hours of standing thus acidulated. Sul- 
phuric acid is more than twice as potent, and usually 
in the strength of one part to five hundred of water 
will render this free from living germs. Sometimes, 
however, it has to be used still stronger, as it is in a 
certain extensively advertised nostrum, in which, 
under a fanciful name, it plays the principal part. 
Alum, lime, and the other chemicals which, by in- 
ducing sedimentation of organic and mineral con- 
stituents, carry down many of the bacteria in water, 
do not (as is often believed) insure the annihilation 
and destruction of organisms present.1 It is the me- 
chanical action of the film, which the presence of a 
1 Residents of the Mississippi Valley, where the principle of set- 
tling water by adding alum has been extensively made use of, have 
expressed to me their belief that many digestive irregularities are 
caused by the use of alum. The danger lies in the use of an ex- 
cess, and probably does not exist with the small amount which is 
permissible (2: 100,000). 
As was just noted of absolute alcohol, Koch also found that the 
spores of the B. anthracis were not destroyed by prolonged ex- 
posure to the action of a four per cent, solution of alum (or a con- 
centrated solution of chloride of lime). 12 
CURRIER, 
minute amount of these causes to settle sooner thaii 
would otherwise occur upon sand in certain large 
sand filters, which insures their excellence, and the 
scarcity of bacteria must here not be attributed to a 
coarse chemical action. When chemicals are added 
in sufficient strength to destroy speedily all the bac- 
teria which may be present, the water thus treated 
cannot be recommended as a beverage. 
With the imperfection of all other methods, we 
have in prolonged boiling a sure disinfectant of any 
water. I have usually found that maintaining a 
water at the boiling point for even less than fifteen 
minutes sufficed to prevent any of its germs from 
developing in gelatine, and this even with bad waters1 
1 The employment of silk threads, in and upon which are dried 
living bacteria with spores, seems to endow these germs with 
greater resistance to the action of steam heat than when the bac- 
teria are <as in drinking-water) each surrounded by water. Hav ■ 
ing tested various waters from different cities, I was lead to believe 
that heating for a very short time sufficed to sterilize them. So I 
have made several series of experiments on the spore-bacteria usu- 
ally regarded as most resistant. I used Croton water, sterilized 
and not sterilized, and bacteria from pure cultures on agar and 
potato (where the spores were, just before the experiments, seen 
by the microscope to be very abundant). I added from these cul- 
tures a small amount to water in flasks. These flasks were well 
agitated several times; then several plates made to test the pres- 
ence of living bacteria. The water was from these poured into 
sterile tubes and Erlenmeyer flasks plugged with cotton. Several 
of these were tested before and after the heating. A wire basket, 
holding eight of these cool test-tubes or flasks (each having from 
ten to twenty c. c. of the infected water), was placed in a heated 
steam-box. The temperature ranged from 930 upward, but the 
thermometer did not register above ioo° C. These tubes were 
taken out as rapidly as I could cool the plates at once prepared 
from them (at intervals of from three to four minutes). While 
this is not so absolute as are some other methods, it seemed the THE EFFICACY OF FILTERS. 
13 
from sewage-infected rivers. The process of distil- 
lation likewise sterilizes water. 
That freezing temperatures destroy only a certain 
proportion of the bacteria which exist in the water 
from which the ice we use is formed, must, I think, 
be evident to those who have read Dr. Prudden’s 
masterly paper on this subject. He there shows that 
the typhoid bacillus in ice retained its vitality for 
one hundred and three days. As he will discuss the 
subject this evening, I shall simply echo his previous 
warning against putting ordinary ice into water which 
we are to drink, as many kinds of bacteria may be 
thereby introduced.1 
best for the practical object aimed at. In this way I found that 
the spores of B. anthracis failed to develop on gelatine (in five 
days) after they had been boiled in clear water for from two to five 
minutes. That this result was not extraordinary will appear to 
those who consult a remark by R Koch (and Gaftky and Loffler) 
in Mit'theilungen aus dem Gesundheitsamte (1881), p 322. 
My results with B. subtilis are likewise not in accordance with 
the accepted views. I am still experimenting in this direction, but 
have never found any water bacteria (which would develop in nu- 
trient gelatine) after the water in which it was had been boiled for 
even a tenth of the time (six hours) which Globig states as neces- 
sary to kill (by steaming) the most resistant red-potato bacillus with 
which he experimented (Globig; Ztschr. f. Hygiene, iii., 2, p. 231). 
He used silk thread wrapped in filter paper. 
1 Prudden; on Bacteria in Ice, etc., N. Y. Med. Record, March 
26 and April 2, 1887. 
Bordoni-Uffreduzzi (Centralbl. f. Bakt. u. Parasitenkunde, i., 
2 Band (1887), p. 495) insists that in natural (Italian) ice the 
bacteria which survive the extensive destruction caused by the cold 
of freezing do not gradually decrease, but that they indefinitely 
retain their vitality. He states that he agrees with Dr. Prudden's 
results in that disease-producing forms of bacteria resist freezing 
better than the harmless ones. 
Frankel—Bakt. des Eises—Zeitschrift f. Hygiene, i., 2, p. 302. 14 
CURRIER, 
The mechanical separation of impurities is the 
principal action in all filters, whatever their composi- 
tion. The amount of oxidation produced by the 
passage of considerable quantities of water through 
inconsiderable masses of charcoal, sand, or anything 
else, seems never sufficient to chemically purify an 
infected water, although the Berlin system of filtra- 
tion, by large beds of sand skilfully managed, effects 
a notable improvement in this respect. 
Almost all tissues tend to separate visible particles 
from water which is turbid, or in which a deposit 
settles. The fact that such matter is retained from 
the water passed through filters appeals very strongly 
to most persons ; and the general assumption that 
such visible matter is dangerous, and that its removal 
insures the purification of water, seems to explain the 
extensive use of filters. They are all composed of 
felt-like matter (filtrum), or of any tissue or sub- 
stance so woven, composed, or arranged as by its 
structure to more or less completely arrest the pas- 
sage of sediment and the floating particles existing 
in varying amounts in nearly all natural waters. The 
filters in domestic use may be ranged in two classes. 
In one class the water runs or is poured into a tank 
or other receiver over or beside some filtering med- 
ium, through which the flow is into a receptacle, and 
from this the water is taken as needed, or is stored 
as fit for drinking. Those of the second variety are 
designed to connect with the faucets, and to strain 
from the water such impurities as it brings through 
the hydrant. 
Or.e of the most widely used varieties is this simple THE EFFICACY OF FILTERS. 
15 
and portable one, which renders water clearer and 
more agreeable to the eye. It is composed of a 
cylinder of pressed carbon, three inches or less in 
diameter, and having nearly the same depth, which 
has from its interior a tube passing through the bot- 
tom of the funnel in which it rests, and thus requir- 
ing all the water passing out of the bottom of the 
funnel first to go through the mass of carbon. The 
flow is very sluggish unless use be made of pressure 
or suction, as, for example, by lengthening the tube 
taking water out of the filter, which impairs the effec- 
tiveness of this so far as purification of the water is 
concerned. After this has been used for a few days 
the retardation is very marked, and is due to the 
clogging of the filter by the various matters which 
form the sediment of the unfiltered water. This 
necessitates frequent cleansing. 
Taking a freshly sterilized carbon filter of this sort 
and pouring into the funnel Croton water just drawn 
after considerable has run from the faucet, and then 
collecting in sterilized test-tubes the first flow from 
the filter, adding of this (as already explained) one 
cubic centimetre to ten parts of softened nutrient 
gelatine, the mingled gelatine and water being 
poured upon a cool plate, it is seen that, before 
three days have passed, several colonies have de- 
veloped. Comparing the number of these with that 
found from the original Croton water plated just 
after being drawn, we find that at least from five to 
eight per cent, of the bacteria pass through the 
filter and develop. The percentage rises as more 
and more water flows through. From fourteen trials 16 
CURRIER, 
made with simple Croton water, I found that through 
a carbon filter, freshly sterilized and the introduc- 
tion of germs from the air and other sources of con- 
tamination being guarded against, the first flow 
showed only a small percentage of the bacteria 
found in the original water used. Then I con- 
tinued the use of the same filter without resteril- 
izing, and endeavored in every way to have the 
conditions under which it was tested as much as 
possible like those which would exist in domestic 
use, and the supply of water in the funnel was al- 
lowed to become exhausted at times, as usually 
during the night. 
In this filter thus used, the number of bacteria in 
the water passing through is, on the second day and 
thereafter, much larger than in the water taken after 
being poured into the funnel and before it has per- 
colated through the carbon block. This excess, 
when large, seems to bear an inverse ratio to the 
volume of the flow. 
It is a familiar fact that, if fresh hydrant water in 
a flask covered with cotton (to prevent the entrance 
of bacteria from the air) be allowed to stand for a 
day at the temperature of the laboratory, the origi- 
nal bacteria in the water may be found to have in- 
creased considerably. So, as a collateral test, after 
water had been poured into the filter and before it 
had passed through the carbon, I have taken some 
of this original water and allowed it to stand in 
test-tubes and flasks, both stoppered and open, by 
the side of the filter similarly protected from the 
entrance of germs from the air, and exposed to the 
same variations of temperature. 17 
This water, examined at the same time with that 
coming out of the filter, showed that the bacteria 
appear to multiply much more rapidly in the sub- 
stance of the filter than outside in separate flasks, 
likewise kept at the varying temperature of the labo- 
ratory, as it then was. The water poured in rarely 
contained as many as 225 bacteria in each c.c., and 
yet in from one to three days from the time when 
the freshly sterilized filter had begun to be used, the 
water after passing through the carbon showed sev- 
eral thousand in every case, and at times more than 
25,000 bacteria in each c.c. 
All sorts of bacteria appeared to pass through, and 
not alone water species. The bacillus of typhoid 
fever will not only pass through, but in two of my 
trials I found that it had increased. For such 
special tests I used sterilized Croton water, into 
which bacteria1 were introduced from pure cultures. 
For artificial typhoid-water the bacillus of typhoid 
was added from agar and potato cultures. Harm- 
ful bacteria can pass through such filters, can pos- 
sibly increase in them under certain favoring con- 
ditions, and from parallel tests seem to retain their 
vitality longer when in the substance of the filter 
than when in a glass vessel beside it. 
Clean sand fairly represents the granular masses 
of filtering material, and has all the advantages of 
THE EFFICACY OF FILTERS. 
1 B. fluorescens (Oneida) and B. liquef. alb. aq. (Croton) 
For cocci: Staph, pyog. aureus and a coccus obtained from the 
water of Salmon River, west of the Adirondacks. For a spiral 
form: Sp. tyrogenum was used as being harmless, yet similar to 
the cholera microorganism. 18 
CURRIER, 
powdered charcoal and magnesia as generally used 
for filtering considerable quantities of water, while 
it is more readily cleansed of the accumulated or- 
ganic matter and, unlike spongy iron,1 adds nothing 
objectionable. Unless, as in the case of the large 
Berlin filters and others acting similarly, rendered 
more effective by overlying silt and organic matter, 
beds of sand do not prevent the passage of large 
numbers of bacteria. I have found many bacteria 
in waters from various artesian wells coming from a 
deep stratum of sand. Through a thin sterilized 
layer, from six inches to one foot in depth, of the 
finest sand procurable from beaches, all the way 
from forty per cent, to ninety-five per cent, of the 
bacteria in the original water pass, even when the 
level of the water is never more than three inches 
above the surface of the filtering mass. When a 
higher pressure is afforded by the water, very few of 
the bacteria are kept back. 
As in nearly all other filters, so through a layer 
of sand all kinds of bacteria, harmless or harmful, 
appear to pass easily. Here, too, they can multiply 
in the moist sand. Thus, I have found the hydrant 
water, which entered the sand filter with 290 bacteria 
in each c.c., to come out through the filter (not 
1 For some of the reasons why spongy iron has fallen into 
disuse, especially in America, see a very instructive paper by the 
ate Prof. W. R. Nichols in the Ninth Report of the Massachusetts. 
Board of Health, 1878, p. 137. 
For favorable notice of spongy iron, see Frankland, Proc. of 
the Royal Soc., No. 338, 1885. I do not care to consider it here, 
for it is a proprietary preparation as offered upon the market (in 
London), is variable in quality, and never sold here. THE EFFICACY OF FILTERS. 
19 
freshly sterilized before using) with sixty and one 
hundred times as many as entered. In a gramme of 
sand taken from a filtering bed, I have found more 
than 10,000 bacteria. 
The combination of other substances with sand, 
as in numerous small and large filters, fails to im- 
prove the usefulness of either, except in so far as the 
elimination of sedimental and other coarser particles 
is concerned. 
Sponge is often employed for this purpose, as well 
as independently, yet it is incapable of separating 
bacteria from the water strained through it simply, 
or through sand or other substances used with it. 
When a large mass of fine, sterilized sponge is 
closely packed, the first few ounces of hydrant 
water that pass through may have a very small per- 
centage of bacteria, but the proportion soon in- 
creases, while if the sponge-mass be considerable 
and loosely packed it may, even in the beginning 
and under exceedingly slight pressure of water, keep 
back no more than fifteen per cent, of the bacteria 
of the original water. This freshly sterilized sponge 
can at the end of the first day of ordinary use cause 
the water then flowing through it to contain from 
increase ten times as many bacteria as the Croton 
water poured in. The stronger the pressure, and 
the more rapid the flow, the lower is the ratio of 
excess due to increase in the filter over the quantity 
found in the unfiltered supply, which, of course, was 
here, as in all other tests, regularly plated at the 
same time with the filtered water. When the flow 
is sluggish, and when it stops entirely at times be- 20 
cause of all the water supplied having passed through, 
the sponge substance favors the increase of the bac- 
teria to a greater extent than is the case with sand. 
The sponge, within twenty-four hours after steriliza- 
tion may, under these conditions, cause the water 
first running through after the intermitting of the 
flow to have five hundred times as many bacteria in 
each cubic centimetre as are found in the water sup- 
plied for filtration. 
The only way really to cleanse such a filter is to 
remove the sponge and boil it. Then the process 
repeats itself. In a few hours after unsterilized 
water moistens it, the mass of sponge is again teem- 
ing with bacteria. Sponge filters are, so far as I 
have seen, becoming less popular. It is easily rec- 
ognized, even by an untrained observer, that the 
organic matter in sponge can undergo decomposi- 
tion under the conditions obtaining in ordinary 
filters. Several of the large filter systems used in 
manufactories, hotels, and other places where large 
amounts of strained water are used, employ sponge 
and the filtered water abounds in bacteria. 
Filter paper, as used in laboratories, is useful in 
separating precipitates and sedimental particles from 
water, but at least from fifty to seventy per cent, of 
the bacteria in Croton hydrant water go through 
with the water filtered, even if the pressure be ex- 
ceedingly slight. The paper which I employed in 
the trials was, of course, carefully selected, and the 
folding was cautiously done, so as to prevent any 
break in the substance. Only single sheets were 
tested. 
CURRIER, THE EFFICACY OF FILTERS. 
21 
“ Prepared cotton,” cleansed as for surgical dress- 
ings, and so made absorbent, removes considerably 
fewer of the impurities of the water than does the 
filter paper. Two-thirds of the bacteria pass through 
when the cotton is at its best, freshly sterilized and 
carefully packed. If enough be used, it will usually 
render the water clear; but, as it has been lauded as 
a filter, I ought to add the statement of my obser- 
vation that, when left moistened with water, as in 
the intervals of filtering through it, the bacteria of 
the original water can multiply over one hundred 
and fifty times, and all kinds can pass through it. 
Combinations of the various substances mentioned 
have hardly any enhanced merit as strainers, and, as 
far as the bacteria are concerned, the combination 
seems to add no safeguard. Sand with carbon 
(bone charcoal) and sponge or cotton, hold back 
at first a larger percentage than sand would, by 
itself. Yet bacteria pass through in any case, and 
the increase afterward is greater than in simple sand. 
This I have seen in various filters. 
In a well-known bottling establishment of New 
York, where immense quantities of artificial waters 
are prepared every week, all the water used in their 
preparation flows through a layer of fine sand in the 
bottom of an enormous vat and, from the receptacle 
into which it then flows, it passes through a layer of 
charcoal. These layers are each less than a foot in 
thickness, and water passes rapidly through. The 
sediment is thereby removed, but in the single ex- 
amination which I was enabled to make I found that 
each cubic centimetre of the filtered water had over 22 
CURRIER, 
3200 bacteria on each of the two plates made, while 
the Croton taken from the hydrant in the immediate 
neighborhood on the day before averaged a few over 
one-eighth as many (432). 
Asbestos of the best quality, new, freshly steril- 
ized and tightly packed, I have found capable of 
holding back all bacteria when the pressure of the 
water was low, and the few that were forced through 
when the substance was defective did not seem to 
multiply so rapidly as in other filter-masses. While 
it, when of the best finish and most tightly com- 
pressed, deserves to rank next to the principle em- 
ployed in porcelain filters, these latter are superior 
in that they can be regularly furnished of a uniform 
and definite quality which produces the best results, 
while I have found it difficult to keep the best 
asbestos boards up to their original standard of ex- 
cellence, and if the surface be large enough to in- 
sure an abundant flow of water, flaws are liable to 
arise, and these let various bacteria through. With 
asbestos it is the fineness of the surface which the 
original water encounters that is important more 
than the thickness of the layer.1 
Although wire cloth strains out the coarser parti- 
cles, its structure does not cause the removal of any 
of the bacteria from water passing through. Still it 
has the positive merit of being easily cleaned, and 
can be heated in an oven or boiled to sterilize it. 
Furthermore, it does not furnish a filter-mass for 
the bacteria to multiply in, although harmful bac- 
1 Hesse; Deutsche med. Wochenschrift, 1885, No. 5. THE EFFICACY OF FILTERS. 
23 
teria can adhere to it and become dangerous if it be 
left uncleaned. 
Closely woven cloth, such as thick, dense flannel, 
when only a slight pressure is exerted by the water, 
may stop ten per cent, of the bacteria in the water 
poured upon it. It is easily cleansed and, if very 
often changed, and boiled before using, it serves 
excellently as a simple strainer without the disad- 
vantage, in this respect, of ordinary filters, in whose 
substance the increase of bacteria may be enormous. 
Like porcelain, filters of porous stone, if of the 
best quality, prevent the passage of bacteria with the 
water filtered through them just after they have 
been sterilized. To test porous stone I employed 
selected perfect specimens of a popular filter which 
very satisfactorily clears all turbidity from as bad a 
water as that of the Hudson River at its worst. In 
these, the stone layer was cemented into a porcelain 
cylinder and was about three-eighths of an inch in 
thickness. The pressure was never more than that of 
ten or eleven inches of water, and, at the fastest, less 
than three-fourths of a litre passed through in an 
hour. As the sediment of the water settled upon 
the surface of the stone, the flow became much 
slower. During twenty-six trials of this kind of 
filter, it had previously been sterilized ten times by 
exposure to moist or dry heat, or to both, for sev- 
eral hours. In every one of these ten cases the 
various waters poured into the receptacle above the 
porous stone for the first time after sterilization, 
flowed through germ-free; that is, the stone was 
permeable for water, but at first allowed no bacteria 24 
CURRIER, 
to pass through, and the plates of gelatine developed 
not a single colony from the usual cubic centimetre 
of the filtered water. After some hours of use, a 
few bacteria had insinuated themselves into the 
stone or were drawn through with the water. With- 
in twenty-four hours the water flowing through and 
collected in sterilized receptacles contained many 
bacteria, which numbered regularly from seven to 
fourteen times as many as were in the original 
hydrant water supplied during these trials. After 
several days, the number of bacteria had so multi- 
plied in the stone, that the first water running 
through after the stone had remained for several 
hours without any flow through it (as happens con- 
stantly over-night in domestic use), showed in each 
cubic centimetre of this filtered water over one hun- 
dred times as many as the average water poured in 
for filtration. To test further these filters, I have 
used artificial waters made by adding bacteria of all 
shapes and characters, from pure cultures to Croton 
water sterilized by boiling. These various kinds 
can get through, but the disease-producing and 
spiral forms usually died out in large numbers be- 
fore any could pass. From Croton water, the most 
marked by its predominance in the filtered product 
was a short, mobile bacillus which was unlike the 
two varieties first forced through the porcelain in 
my experiments. 
Allied to the porous stone filters are those of 
porous porcelain prepared in this country. Porce- 
lain baked at the same low temperature as the porous 
cups used in electric batteries is, in its efficacy, very THE EFFICACY OF FILTERS. 
25 
much like the stone just described. Under a low 
pressure, water passing through this substance during 
one or two days can become completely sterilized, 
which is a result superior to that got from the stone 
filters. Before much water has passed, the bacteria 
make their way through and multiply somewhat on 
the passage. This material, as I have thus far found 
it, of American manufacture, is therefore imperfect, 
yet is superior to rebaked porcelain in the single 
important respect that it is not brittle, while the 
rebaked porcelain cylinders of the Chamberland- 
Pasteur filters are quite fragile.1 
Recognizing the defects of other substances, Pasteur 
and Chamberland2 called attention to their filtering 
cylinders, in which the important element is unglazed, 
rebaked porcelain. Originally employed to facili- 
tate “ cold sterilization ” of fluid culture media, the 
fact that water went through the porcelain without 
carrying any bacteria with it has caused attempts to 
popularize its use. Inasmuch as some bacteriolo- 
gists on both sides of the Atlantic had reported that 
this filter prevented the passage of bacteria, I was 
careful to use only selected cylinders without flaws, 
1 It has for some time been known that fluids can be rendered 
germ-free for laboratory use by being passed through solidified 
plaster-of-Paris (best prepared with incorporation of asbestos 
shreds). These filters, however, lack permanency and cause the 
objectionable sulphate of lime to be present in the filtrate. 
2 Bouley (for Chamberland) : Comptes Rendus de l’Acad. des 
Sciences, 4aout, 1884. 
Chamberland : Comptes Rendus Hebd. (etc.) de la Societe de 
Biologie, 1885, p. 117. 
Immediately following this {idem., p. 120) were reiterations of 
adverse criticisms by Galippe and Bourquelot. 26 
CURRIER, 
and did not omit any precaution to prevent the 
introduction of contaminations. 
The most satisfactory way of using this for testing 
was to employ the single porcelain cylinder, within 
the usual casing, attached to a hydrant system and 
with a pressure of water varying from six to fifteen 
pounds.1 Even with the highest New York hydrant 
pressure the flow is only drop by drop. It can be used 
to siphon water out of a jar, and in this way it works 
nearly as rapidly as when attached to the hydrant in 
the basement of a house in the centre of this city. 
By a porcelain cross-tube sold in Paris, two or more 
cylinders can be siphoning at the same time into the 
same tube. To avoid risk of contamination, I used 
for siphoning a single cylinder, to which a bent glass 
tube was attached by a tightly-fitting piece of rubber 
pipe, all parts having been sterilized as well as the 
water by which the flow of the siphon was started. 
I got germ-free results regularly by this means in 
seven trials with two cylinders used separately;2 but, 
in order to insure this, all the precautions mentioned 
are necessary as, in the water siphoned in domestic 
use of these cylinders, bacteria of various sorts are 
1 This highest pressure was only at night, and once on Sunday 
night the pressure for a short time was three pounds greater. 
During the daytime the pressure was rarely more than ten or 
eleven pounds, but occasionally the gauge vibrated a little higher 
on account of the concussion of the water while a neighboring 
pump was working. The pressure was tested by an expert, and 
was only half of that indicated on the French circular of the agents 
of this filter in Paris as yielding 200 litres (50 gallons) per day. 
2 One cylinder being used three times, and another four times, 
each being carefully sterilized before each trial. THE EFFICACY OF FILTERS. 
27 
found, yet they are possibly contaminations due to 
the inherent defects of all such joints as are used. 
So I have devoted more time to testing these when 
directly connecting with the hydrant, as very slight 
precautions are needed, in their use with faucets, to 
eliminate all chances of contamination. 
My first thirteen trials of this filter used in this 
way were under less satisfactory conditions than the 
fourteenth and those following, as the former were 
in a laboratory where the available water came under 
not much more than twenty feet pressure from a 
tank, and I was not able so well to control the con- 
stancy of this pressure as I was during the later trials. 
My first sixteen tests of various ones of these cylin- 
ders gave sterile water. Then, attached to a base- 
ment faucet, with a direct flow from the hydrant 
under a pressure never exceeding fifteen pounds (ex- 
cept that on Sunday nights it was for a few hours 
three pounds higher), the filter, after several days of 
intermitting use, gave over 3500 bacteria in each 
cubic centimetre and all the water examined (two 
samples per day) for four days showed an increasing 
number of bacteria.1 
On the third and fourth days after the first passage 
of bacteria through the porcelain wall of the cylinder, 
the number in each cubic centimetre varied above 
and below 400,000. The filter was then sterilized 
by steaming for five hours, as was also done between 
1 The varieties first passing through were: 
1. Colonies liquefying gelatine, and composed of medium-sized 
very mobile bacilli. 
2. Superficially-spreading colonies of short, mobile bacilli. 28 
CURRIER, 
the second and third series of tests. These tests 
bore out the results of the former observation. 
That these results show that average specimens of 
this substance have the inherent defect of all filtering 
material, though in a much less degree than all others, 
seems to me unquestionable. That the bacteria did 
not get through owing to some defect in the partic- 
ular filters which I used is evident from the fact that 
the cylinders employed'’(and which were selected by 
me in Paris as perfect), through which the bacteria 
passed, are still free from any appearance of a flaw 
which could explain the passage of these micro- 
organisms. The same filter which (first of those 
used in my tests) allowed bacteria to pass through, 
has repeatedly filtered water free from all bacteria for 
a few days after each of the several sterilizations 
which it has since undergone. 
In three following series of trials of this identical 
cylinder, bacteria have gone through its walls in 
from three to four days after each sterilization. 
They have also gone through a second perfect cyl- 
inder with which I experimented. That the bacteria 
were not introduced by careless manipulation is 
evident by reason of their number, and because of 
the fact that, after the filter was sterilized, they were 
found in each case at a particular phase of each 
series of tests with the intermittent pressure which 
only for a few hours once a week was as high as 
eighteen pounds, and was usually as low as eleven. 
At the beginning of each series of tests the filter was 
sterilized. 
The bacteria were not splashed in from the sink 
below, for, in the first place, all splashing failed to THE EFFICACY OF FILTERS. 
29 
carry in any as revealed by testing the water filtered 
after such experimental attempts to throw water in, 
and none had ever multiplied inside or even re- 
mained there during my first thirteen tests, when 
there was much splashing in the common sink. Dur- 
ing my later trials there was no chance of such con- 
tamination. 
To answer fully in advance any such objection, I 
will state that after I got the enormous number of 
over 400,000 in each c. c. flowing through the por- 
celain, and had sterilized the cylinder, water was 
allowed to flow through it and plated, but was sterile 
in that no bacterial colonies developed. Then, while 
the filter was in action, I introduced, with a sterile 
needle (from below through the aperture into the 
inside of the cylinder), and, touching the needle to 
various parts of the surface, deposited over it pecu- 
liar bacteria from a pure culture derived this winter 
from the water of one of the northern lakes which I 
examined during my investigations as biologist for the 
Syracuse Water Commission. The colonies of this 
bacillus, when growing on gelatine plates, have a 
peculiar odor, a distinctive green color, and other 
characteristics which clearly differentiate them from 
any bacteria ever found in the Croton water. The 
water flowing out all the while, or rather dropping 
out, was, three minutes after the introduction of these, 
collected in sterilized test-tubes and was found to 
contain 175,000 of these peculiar bacilli in each c. c. 
After seventy-five minutes there were only 14 bacilli 
in each c. c. of the water then flowing through the 
porcelain and two hours later, and during the next 30 
CURRIER, 
day, there were no bacteria of any kind in the water 
which again later in the series showed that the Cro- 
ton bacteria had passed through with it. This proves 
that the increase in the number of the bacteria was 
not after the water had passed through the substance 
of the filter. 
These facts, together with the extreme fragility of 
this rebaked porcelain, and the exasperating slow- 
ness of the flow, which does not improve on long 
trial, have caused me to modify the recommenda- 
tions which 1 gave among my colleagues and others 
after my first tests of these filters. 
As regards rapidity of flow and non-fragility, a 
notable improvement over the cylinder form (which 
I have just spoken of) is afforded by a series of two 
or more flat plates of porcelain similarly prepared. 
In the Varrall-Brisse pattern these porcelain plates 
are backed by carbon plates of the same shape and 
size and the filter yields sterile water for a long time 
under gentle pressure, although I do not consider 
the carbon an advantage. The further such filters 
vary from the simplest type, the greater is the likeli- 
hood of the introduction of contaminating bacteria 
into the filtered water.1 
1 As out of twenty-seven kinds of filters that I have been able to 
test, this seems, on the whole, the best substance yet produced 
for the purpose, it is to be hoped that American ingenuity make 
further improvements upon the models now existing. The cost 
need not be great even if all be carefully tested before being sold, 
as suggested by Fol and Dunant (Rev. d'Hygiene, 21 mars, 1885, 
p. 183). 
The “ Pasteur Filter Co.” requests me to state that it now fur- 
nishes a slightly different style of cylinder in which the porcelain 
is invariably of fine quality and claimed to be “germ-proof.” THE EFFICACY OF FILTERS. 
31 
Of the various forms of filters used upon faucets, 
I feel bound to warn against those rotating within 
an outer shell. Whether packed with charcoal, sand, 
or any other material, and even if the water go 
through the filtering mass and not around it, the 
filter has the great drawback of various irregulari- 
ties and depressions on the interior which retain 
organic matter and allow bacteria, both harmful and 
harmless, to increase in it and maintain their vitality 
for a longer time than in ordinary filters. This con- 
clusion I am led to by the results of various experi- 
ments and observations. In a recent epidemic in a 
large New England city this form of filter appeared 
to play an active part in continuing the infection, 
for cultures, in every respect like those of the germs 
of typhoid fever, were derived from several of these 
suspected filters by Dr. Prudden and others. I have 
found that the typhoid bacillus lives much longer in 
the sediment collected from filters than in pure 
water.1 At its best, this filter cannot remove the 
11 have removed the slimy sediment from the outside of a porce- 
lain filter, and without sterilizing this, put it into sterile test-tubes, 
and then added water in which the germs of typhoid were found 
living. After these had stood together for four weeks, several 
plates of the water were prepared and, making several cultures 
from the various colonies which appeared like typhoid, a bacillus 
identical in all its features with that of typhoid was isolated, and 
considered as typhoid by those to whom it was shown. This is 
longer than I have been able to keep this bacillus alive in pure 
Croton which had been sterilized. All waters tested were in tubes 
of the same size and at the same temperature. In laboratory ex- 
periments we are not able to reproduce the conditions existing in 
the large bodies of water, where these harmful bacteria can prob- 
ably live longer still under the evener temperature there prevail- 
ing. 32 
bacteria from water, and they multiply in the char- 
coal or sand so that, even after several minutes of 
washing out by the stream flowing from the hydrant, 
the water coming through this filter may show sev- 
eral times as many as are in the water directly from 
the hydrant. To avoid waste of time in repetition, 
I will refer to the facts observed with the sand filter, 
as the mechanical action of the filtering mass is iden- 
tical in both. 
The sand filter, for attaching to faucets, is perhaps 
the simplest and most popular used. It consists of 
a cylinder of like breadth and length, being not 
quite three inches long. It screws by either end upon 
ordinary faucets, and its interior is filled with crushed 
silica or fine sand, kept in place by wire-gauze at 
either end. I have adapted several much larger 
without securing more satisfactory results. Taking 
a new one, sterilized throughout, allowing a fair 
stream of water to flow through it when attached to 
a faucet and at the same time taking water directly 
from the same supply-pipe from the hydrant by 
means of another faucet by its side, and under the 
same conditions of pressure and outlet, except that 
there is no filter upon the second faucet, the water is 
seen to flow in a free stream nearly as large as from 
the unobstructed faucet, while the Pasteur filter, at- 
tached to the same supply-pipe and with the same 
pressure, allows the water to come at the rate of only 
a few drops each minute. Even with a gentle stream 
through the freshly sterilized sand filter, very few of 
the bacteria are arrested. With very fine sand in a 
cylinder holding three times as much as the ordinary 
CURRIER, THE EFFICACY OF FILTERS. 
33 
one, and a very small stream flowing, I have ob- 
served more than fifty per cent, to be retained. 
This result is seen only in the first use of an abso- 
lutely sterilized filter. 
Speaking of the tank variety of sand filters, it was 
shown that moistened sand favored the increase of 
bacteria brought into it by Croton water which had 
flowed through. In the sand or other granular con- 
tents of these filters attached to hydrants the same 
multiplication takes place when the water has for 
some time ceased flowing, and this increase is at a 
more rapid rate than in water standing in a clean 
vessel by the side of the filter. The first flow in the 
morning, for instance, after the filter has remained 
unused over night, shows a much larger number of 
bacteria than the hydrant water then contains. 
Even after the water has flowed for several minutes 
for the purpose of washing out the filter, the bacteria 
are in excess over the number in the original water. 
After such a filter has had the hydrant stream run- 
ning through it long enough to wash out all the 
germs that have resulted from the increase (which 
complete cleansing is not always producible), the 
bacteria in the water may appear nearly the same in 
number as in the hydrant water, varying slightly 
either above or below. While such filters are worse 
than useless from a bacteriological point of view, 
they are good strainers when not clogged by too 
long use without cleansing. As strainers they are 
less troublesome than flannel bags (which are as safe 
as these sand filters for use with suspected water, 
provided that they are removed at least once a day 34 
CURRIER, 
to be disinfected by boiling). The act of reversing 
this popular sand filter does not cause the removal 
of the organic matter upon which the bacteria can 
be nourished. Only the loose outer portion is 
thereby removed. Such a sand filter is a safer 
strainer when never reversible and it should be 
cleansed at least once a day when needed at all. 
Powdered magnesia and other granular substances 
used instead of sand in the style of filter just de- 
scribed, have no superiority over the easily cleansed 
sand. 
Asbestos-board I have not been able to procure of 
sufficiently firm, strong, and fine surface to resist the 
passage of bacteria under any considerable pressure 
from the hydrant brought to bear upon such a sur- 
face. The quality of the surface seems the impor- 
tant point. To yield a flow of any degree of rapidity, 
this surface must be large and that creates a tendency 
to break and necessitates a bulky filter, which most 
people shun. Packed asbestos shreds allow a few 
bacteria to pass through when the sterilized filtering 
mass is so dense as to cause the flow to be very slow. 
In this substance I have, as already mentioned, not 
observed so marked multiplication of bacteria as in 
the sand, porous and other filters. 
Although large bodies of sand fail to prevent the 
passage of bacteria with the water which percolates 
through, it is noticed that certain natural filters of 
this sort render water much freer from bacteria when 
the water has first to make its way through a layer 
of silt and minute particles which have been arrested 
by fine sand. It is from the application of this THE EFFICACY OF FILTERS. 
35 
principle that the large filter-beds are built which 
purify the water distributed by hydrants throughout 
various cities. Taking as a model the very carefully 
managed filter-beds of Berlin, it is seen that above 
the base of the filtering tanks is a layer of a foot of 
stones, gradually becoming smaller in size toward 
the upper surface, upon which is coarse gravel to 
the height of a foot more, then upon this a little 
more than two feet of sand, which at the top is as 
fine as can be procured. When the filter-bed has 
been freshly cleaned, as is found necessary for it 
after being constantly used for a week or so (occa- 
sionally several weeks), purified water is slowly 
backed into the filtering-mass from below until this 
water, carrying up all the air with it, has reached 
the top of the upper layer. Then the ordinary 
river (or lake) water is made to flow very gently in, 
to the depth of one metre. This is then allowed to 
stand for at least twenty-four hours. The nitro- 
genous or other particles, confervoid vegetation, 
and whatever else the water contains as sediment, 
have then settled upon the upper portion of the fine 
sand without sinking deeper, and a delicate film is 
formed which, with careful inlet and gentle pressure 
(never to exceed two metres of water), retains nearly 
all the bacteria of the water supplied, and prevents 
their passage, provided that the flow through is very 
regular and slow (never more than three metres 
in a day). Nearly all this separation of the bacteria 
is produced by the sedimental matter retained on 
the surface of the sand, so that, when the filter slows 
from clogging, it is found that less than half an inch 
of the upper layer of sand need be removed. 36 
CURRIER, 
From the statistics of Wolffhiigel1, and of Plagge 
and Proskauer,2 we learn that the number of bac- 
teria, originally very large in the river Spree, is re- 
duced to a very small percentage in the filtered 
water, oftentimes a fraction of one per cent. I may 
add that the organic constituents are also decidedly 
lessened, and that the oxidibility, as tested by the 
reduction of a standard solution of permanganate of 
potash, is seen to be regularly lowered one-fourth 
and often one-third of the amount shown prior to 
the filtering. The long arrays of figures given in 
the reports of the action of these filter-beds show 
that, with hardly an exception, the ammonia is con- 
stantly removed by the process. 
In this country the filter-beds, all of which are 
built upon the same plan as those in Berlin, are less 
perfect in their action, as the people in charge do 
not devote sufficient care to sedimentation in the 
case of those which I am familiar with. Thus, a 
very carefully planned filter-bed, which filters river- 
water for one of our cities, allowed, at the time 
when I examined it, nearly half of the large number 
of bacteria in the water pumped from the river to 
pass through. This unsatisfactory result seemed to 
be due to the fact that too much water was required 
from the small filter-beds and hence not enough 
care could be bestowed upon the thoroughness of 
the filtration. 
Of the various large filter systems which I have 
tested while they were in use in various manufactories 
1 G. Wolff hiigel: Arb. aus d. Gsdamt., ii. i und 2, p. 109. 
1 Plagge u. Proskauer. Zeitschrift f. Hygiene, 1887, ii. p. 401. and institutions, for which these filters were straining 
large quantities of water, the only kinds which re- 
duced materially the number of bacteria as found 
in the hydrant-water were those in which, by the 
addition of a minute proportion of a suitable salt, 
usually alum (as approved by the general experience 
of years in many districts), the suspended particles 
with the organic matter are caused to settle more 
rapidly, and then these form a deposit upon very 
fine sand, through which water is forced. From sev- 
eral examinations of a filter of this type, which in 
every case rendered the water much freer from germs 
than the original (unfiltered) water, I once found the 
filtered water wholly sterile, while the (Brooklyn) hy- 
drant water of the neighborhood at the same time 
showed 397 germs developing from each c. c. This 
particular filter was constructed on the principle of 
Prof. A. R. Leeds, who informs me that the alum 
was present in the proportion of only i part in 100,000 
of water. I could not, by the ordinary tests, detect 
the presence of this salt, and such a small amount 
can cause no appreciable harm. The alum used is 
probably decomposed, and its elements taken up by 
the organic matter and bases present. As previously 
stated, the sterilization of the water is here caused 
by mechanical and not by chemical action. In my 
tests the typhoid bacillus of the first agar culture 
from a recent spleen was not destroyed by standing 
for forty-eight hours in a solution of ten times the 
strength of that employed in these filters, while many 
other bacteria were alive after standing for forty- 
eight hours in an alum solution of 1 part to 1000 
THE EFFICACY OF FILTERS. 
37 38 
CURRIER, 
of water, which sufficed to destroy the typhoid ba- 
cillus in that time. 
In the present stage of chemical knowledge there 
seems no sufficient evidence that in water these mi- 
croorganisms evolve the organic alkaloids which are 
considered to be produced when harmful bacteria are 
introduced into the system and by their action there 
cause disease. What the exact nature of the vola- 
tile organic constituents carried over in distillation 
may be, and whether they be deleterious or not, 
rests for the science of the future to determine. 
The process of boiling previous to vaporization in 
the still must kill all harmful bacteria, and all other 
bacteria that I have ever encountered in the original 
hydrant water from which such water as the well- 
known “ Hygeia ” is prepared. The bacteria which 
I have found in specimens of this excellent distilled 
water that I examined must have been introduced 
from without after the water was condensed from 
the still and before the bottling process was com- 
pleted, and in this I have never found any other 
than harmless varieties. Water condensed from va- 
rious stills of approved patterns I have found sterile. 
The relatively large amount of free ammonia found 
in such distilled waters (it being several times greater 
than any detected in the Croton hydrant water) does 
not seem to me to be objectionable. The condensed 
steam produced in distilling, and which constitutes 
the Hygeia water, represents a much larger original 
amount of water than results after the process of 
vaporization and condensation is completed, and the 
ammonia may besides come from the air and from 
other sources. THE EFFICACY OF FILTERS. 
39 
To those who desire a water freer from alkaline 
earths than the ordinary “hard ” water which they 
may have to drink, it should be mentioned that the 
process of heating, in the sterilization by boiling 
which I so urgently recommend when it is necessary 
to drink suspected water, drives off free carbonic acid 
and so causes the deposition as an insoluble sedi- 
ment of the carbonate of lime and other objection- 
able salts which form the so-called “ temporary hard- 
ness.” The water is thus rendered “ softer.” 
When boiling is resorted to because of actual in- 
fection of the water, it should be carefully done. 
It is best to have the water, perhaps after a prelim- 
inary boiling and straining, decanted into bottles, 
each holding say a quart, and these covered with a 
suitable glass cap or plugged with cotton or pro- 
vided with a clean stopper loosely dropped in, and 
then the bottles placed in a kettle or cauldron in 
the absence of a steaming apparatus. This is heated 
and kept near the boiling point1 for an hour. Then 
the whole is allowed to cool, and the bottles may be 
placed near ice; but into the water thus carefully 
prepared it is not advisable to put ordinary ice, for 
harmful bacteria may retain their vitality in ice 
1 Although Miquel and others have called attention to micro- 
organisms retaining their vitality at a temperature above 710 C. 
(]6o° F.), these seem to have no harmful character. See Glo- 
big (Ztschr. fur Hygiene, iii., 2, pages 322 and 331). The spores 
of the “red potato bacillus,” which he reports as very resistant 
even to steam heat, have never appeared in waters that I have seen ; 
and the time limits for boiling which I have elsewhere given, after 
careful observation, hold true as regards all known disease-pro- 
ducing bacteria. 40 
CURRIER, 
longer than in water. Pure water loses all its excel- 
lence when unwholesome ice is melted in it. For 
those to whom the taste of boiled water is unpleas- 
ant, weak teas, wine, or other savor may be added 
to mask the absence of the more agreeable taste im- 
parted by the process of aerification (which under 
ordinary conditions may itself be a means of adding 
bacteria to the water).1 
There are ingenious hypotheses as to the utility 
of nitrifying bacteria in overwhelming other varieties 
and conjectures that, if recognized and planted upon 
filters, these nitrifying ones would safely destroy the 
pernicious ones; but among these theories I know 
of none to be seriously recommended. The recent 
results in America and various parts of Europe are 
so inconclusive and diverse that the brief limits and 
practical aim of this paper will hardly permit any 
further consideration of them. 
Summary.—Boiling sterilizes water, and within 
thirty minutes will have killed harmful bacteria. 
Drugs and other agents acting chemically, if used 
1 Berckholtz (Arb. aus d. K. Gesundheitsamte, 1889, p. 14) (em- 
ploying exsiccator and four days of agar culture, dried on silk 
thread) found that the cholera spirillum, contrary to earlier results, 
retained its vitality in air for over two weeks. 
In connection with my experiments I have found the typhoid 
bacillus to live dried on the glass of a test-tube, for a week and 
longer. I have made no special attempt to determine how long it 
will live in a dry state. 
Tommasi-Crudeli, Dr. Bern. Schiavuzzi in Pola (Rendi Conti 
della R. Accademia dei Lincei, Dicembre, 1886) say with others 
that the malaria bacillus, which they, like Klebs, found in soil and 
air, may be carried by and live in water. The same is true of many 
other bacteria introduced into water. THE EFFICACY OF FILTERS. 
41 
in amounts which are commonly safe, do not steril- 
ize water. 
The prolonged heat which water undergoes in the 
usual process of distillation destroys all germs which 
may be in the water undergoing the process. 
Ordinary filters, even if satisfactory as strainers, 
fail to remove all bacteria from drinking-water. So 
far from lessening the number in the original water, 
the filtering substance may allow a more rapid mul- 
tiplication than these microorganisms would ordi- 
narily undergo in the unfiltered water on standing, 
and the germs of disease, even if held back by the 
filtering substance, may be harbored in all filters. 
The finer the substance through which the water 
passes, and the lower the pressure, the more perfect 
is the action of the filter in holding back the bac- 
teria. 
Of all substances thus far furnished for domestic 
filters, porous rebaked porcelain, carefully selected, 
I have found to be the best. If thick and strong 
enough to allow the use of a large surface, and the 
substance remain perfect (without flaw or break), 
this may yield a fair flow of clear water, free from all 
bacteria; yet under our ordinary Croton pressure of 
one atmosphere or less, this yield is only in rapid 
drops, unless the apparatus be complex. 
To insure the permanency of this action, the 
filter should be occasionally sterilized throughout, 
by steaming or by other means; for, under prolonged 
pressure, various kinds of bacteria can go through, 
and in the copious organic matter collected on the 
filter some harmful microorganisms can retain a 42 
THE EFFICACY OF FILTERS. 
high degree of vitality for weeks longer than I have 
ever found them to live in pure water. 
Where filtering is really necessary, it is in general 
best for the community that it be done carefully on 
a large scale through sand-beds upon which a fine 
layer of organic and inorganic matter is expressly 
produced by sedimentation, because of its valuable 
action in holding back the great majority of the 
bacteria. 
A bad water filtered is less desirable than a pure 
water in its natural state. When, therefore, filtra- 
tion is employed because of real danger of infection, 
the filtered water should, as a rule, be furthermore 
boiled, as the entire absence of sediment and cloudi- 
ness does not insure that the bacteria of disease may 
not have made their way through the filter.