1. Soap as a Lithographic Material.—Soap consists of an alkali in combination with a fatty acid. The alkalies used in soap-manufacture are soda, potash, and ammonia; the acids are chiefly oleic, stearic, palmitic, and margaric. Soda forms the “hard” soap; potash the “sweet” or soft soap; and ammonia the kind of soap used in medicine, technically called liniment. Soda soaps will vary in hardness according to the acid employed. Stearic and margaric acids yield harder soaps than the oleic and palmitic. Soap, although it is of so much importance, is one of the least reliable compounds which the lithographer has to use. The only advice that we can offer in regard to obtaining an article fit for his purpose, is to apply to a respectable shop, and ask, and pay, for the best. Best white or yellow is what we employ. Seeing that soap may by dexterous management be made to contain 80 per cent, of water, that 20 per cent, may be considered a minimum, and 40 per cent, an average amount, it is no wonder that various results are obtained from apparently the same material. Supposing that it is desirable that soap for lithographic ink should consist of stearate of soda only, there is little chance of obtaining it of pure quality, when various samples of commercial soap are found to contain the following substances: glycerine; silicate, sulphate, chloride, and carbonate of soda; rosin; gelatine; fuller's earth; Cornish clay; ground flints; potter's slip; farina; dextrine, and other substances.