or LITMUS. Lichens belonging to the genera Roccella, Varioharia, and Lecanora furnish the red, violet, and blue colours, commonly known as archil, cudbear, and litmus. The colouring matter does not exist readily formed in the lichens, but they contain from 7 to 12 per cent. of certain colourless acids, such as erythric, eversic, and lecanoric; which acids, when acted on by alkalies, form new acids, and these, on the addition of an alkali, produce a violet colour, and also a colourless neutral substance termed orein. This is the colour-producing body; and when acted on by ammonia, it absorbs oxygen, and is converted into orein, an azotized body with powerful tinctorial properties.

In preparing archil, cudbear, or litmus, the lichens are first cleaned from earthy matter, and are then made up into a paste with an alkaline solution, which in the case of archil is urine, with the addition of lime for separating the ammonia. By exposure for some weeks to the air, with occasional additions of the alkaline liquor, the paste assumes the characteristic colour of the lichen used. In this way cudbear is obtained from Lecanora tartarea, and litmus from the Roccella tinctoria; but in the latter case it is moistened with a mixture of carbonate of ammonia and carbonate of potash, and the colour is first red, and afterwards intensely blue. It is made up into small cakes, with chalk or plaster of Paris. The colouring matter of litmus is soluble in alkaline liquids, but only partially so in water and in alcohol. According to Kane, litmus consists of several principles, one of which is a red semi-solid body, soluble in ammonia with a purple colour, and termed erythroline; secondly, a body named erythrolitmin, which forms crystalline grains of a dark red colour, and is soluble in alcohol, but not in ether; it becomes blue by the action of potash. Neither of these bodies contains nitrogen, but the third and chief component of litmus does so, and hence has been termed azolitmin. It is not soluble in alcohol or in ether, and only sparingly so in water; it has a brownish-red colour. It is soluble in ammonia, when its colour becomes blue; and when this solution is mixed with the salts of some of the metals, blue and violet lakes are formed.

Litmus is much used by the chemist for detecting the presence of acids, which turn it red; the blue colour is restored by the action of alkalies, so that when slightly reddened, it may be employed as a test of their presence. The most convenient method of applying the test is by means of strips of unsized paper, tinged by an aqueous solution of litmus, and then dried.