CD probably began writing on Drosera (sundew) between June and October 1873 (see ‘Journal’ (Appendix II)). In Insectivorous plants, pp. 174–5, he listed the effects of alkaline, earthy, and metallic salts on the leaves of Drosera according to the order given in Henry Watts’s Dictionary of chemistry. CD’s annotated copy (see Marginalia 1: 195–6) consists of the first three volumes of the second edition (Watts 1872–4), volumes four and five of an 1871 reprint of the first edition (Watts 1862–8), and a supplement to the first edition dated 1872 (Darwin Library–Down). The substances CD experimented with included lithium nitrate, lithium acetate, and caesium chloride. For the definition of chemical affinity in this period, see Watts 1872–4, 1: 850–67.