24th March, 1958.
Dear Rosalind,
Herewith the potato virus X.  It contains very little salt and about 6 mg. of virus per ml.   Keep it in the cold but don't freeze it.  As it tends to go insoluble, particularly when precipitated or sedimented, it may not prove a very suitable object for you to study.  Phosphate buffer is not to be recommended and I suggest you use M/20 borate buffer at about pH 7.5.  Even when already insoluble in water it will sometimes dissolve in this buffer, and it stays soluble in this buffer for longer periods than in most other fluids we have tried.  Your type of borosilicate glass should be quite suitable; indeed from what I have already said about borate buffer, it may even be desirable.  Potato virus X is more resistant to alkali than tobacco mosaic virus, and much more so than A-protein.
As there is 40-50 mg. here, there should be enough for you to make several attempts and I wish you the best of luck.
Yours sincerely,
F. C. Bawden