Note 329 — MINERAL POISONS. (continued 2)

There are some exceptions to this law; for instance, the tincture of litmus, and litmus paper, are always rendered more intensely blue, by the addition of alkalies. There are also other bodies, besides alkalies, which change the yellow colour of turmeric to a brown. Upon this subject see an interesting paper in the 26th number of the Journal of Science and the Arts, p. 315, by Mr. Faraday, entitled “On the changing of vegetable colours as an alkaline property, and on some bodies possessing it.” By this communication we are informed that even the strong acids redden turmeric paper, and that a very weak nitric acid gives it a tint exactly like that produced by an alkali. Different metallic salts are characterised by similar effects.