My dear Sir

I am sorry to learn that you cannot send the washings of the Drosera glands earlier than a fortnight hence, because this laboratory closes on the 26^th. inst. & I leave for Switzerland a few days later. I shall be back & the laboratory reopened on the 1^st. of October. Will that be too late?¹

I am inclined to think that your suggestion about the Pepsin is probably correct especially as the secretion is slightly acid. The identification of Pepsin by analysis of washings would be exceedingly difficult if not impossible, because Pepsin is not a definite compound the composition of which is known, it is rather of the nature of a ferment & as such is probably continually undergoing chemical change.²

It appears to me that the most hopeful way of ascertaining if the gland liquid is of a pepsin character would be to operate with the concentrated liquid detached from the hairs & to ascertain l^st. If its digestive powers are obliterated by the neutralisation of its acidity & again restored by the addition of hydrochloric acid. 2^d. If its digestive powers are permanently destroyed by the addition of strong alkalies, and impaired by strong acids. 3^rd. If the liquid is most active at blood heat.³

So far as the washing is concerned I am of opinion that a solution of carbonate of soda is preferable to one of carbonate of ammonia.

Believe me | Yours sincerely | E. Frankland

Charles Darwin Esq. F.R.S.