To Edward Frankland   13 October 1873

My dear Sir

I am quite ashamed of myself at the trouble which I have caused you, more especially considering the value of your time. I did not in the least know what labour it was to test for organic acids.¹ I can only say that I feel very grateful. The negative information is very interesting to me, for to the day of my death I sh^d have been haunted with the belief that the acid was hydrochloric.² It will be extremely useful to me to give your approximate determination of the acids. There can be no doubt that the viscid secretion, whenever the leaves were closely clasped over any object, instantly coloured my litmus paper bright red, for I have tried this many scores of times.³ When a substance like Casein or Chlorophyl is on a leaf, which is not readily absorbed, but excites much secretion, I have found that it has strongly coloured litmus paper for 8 successive days. Although I washed so many leaves & gently scraped them in the Sol of C. of Soda,⁴ yet a large quantity of the secretion still adhered to them, & this may have contained a good deal of the acid.

Unfortunately, before sending you the leaves, I partially dried them between blotting paper.

In my ignorance I am perplexed about the litmus paper which you have so kindly sent me.⁵ It is, as you may see by the enclosed, of a pink colour, but becomes blue if held in the vapour of Carb. of Ammonia. There certainly is no acid vapour in my room; & if the pink colour is not right, I imagine that there must have been a bottle with some volatile acid in the Post bag with your letter; but pray do not give yourself any more trouble about it.

With my cordial thanks | yours very sincerely | Charles Darwin