My dear Sir

I have received Hofman’s letter which is extremely interesting to me.² It is a great shame to trouble you, but I should be very much obliged for a line here, of explanation, if you can give it.

Does Hofman mean that for instance $\frac{1}{\mathrm{64,000}}$ part³ of a grain of arsenic can be detected;—or that $\frac{1}{\mathrm{4,000,000}}$ part of a grain of iodine can be detected; surely that is quite incredible. Or does it mean that in a large body of water containing one part of iodine to 4,000,000 of water, that the starch in the course of time will out of this very weak solution absorb iodine so as to colour it. If this is meant these facts do not, in fact, show how little of these substances produce a visible chemical result.⁴

Pray forgive me, and believe me | Yours very truly | C. Darwin

At present I fully believe after endless experiments that $\frac{1}{\mathrm{64,000}}$ of a grain of C. of Ammonia produces on an absorbent gland of the Drosera a visible effect.⁵