The correspondent is conjectured from the fact that Baxter was CD’s usual chemist, and he supplied chemicals for CD’s work on Insectivorous plants. The year is established by a note accompanying this letter in which Horace Darwin is said to have attested that the letter had to do with CD’s work on Drosera (sundew). CD did experiments on Drosera with litmus paper and bought some from ‘a druggist’ in 1873 (see Correspondence vol. 21, letter to Edward Frankland, 7 October [1873]). He started to use the more sensitive litmus paper provided by Edward Frankland from October 1873 (see ibid., letter from Edward Frankland, 10 October 1873).