In few ways can disinfectants be so agreeably applied to the skin as when incorporated with soap. One of the last introduced, though probably one of the most efficacious, is thymol soap - first made by Ferris & Co., Bristol. Thymol is a non-poisonous (herein differing from carbolic acid) crystal,about 8 times as powerful an antiseptic and disinfectant as carbolic acid, and is probably the only substance that combines disinfecting properties with a really pleasant smell, that of thyme. The best mode of incorporating thymol and phenol (i.e. carbolic acid) with soap is a trade secret; Morfit, however, states that carbolic soaps are best made by his process, using as a basis hot-pressed fat-acid cake, on account of the tendency of carbolic acid to soften the soap-paste.
Carbolic soaps are made in great variety and in large quantities by F. C. Calvert & Co., of Manchester, whose products contain specified definite quantities of carbolic acid of various qualities. Their "medical" soap contains 20 per cent. pure crystallised carbolic acid; their toilet and household soaps, 10 per cent.; their domestic soap, 8 per cent.; and their "No. 5" or "scouring "-soap, 4 per cent. liquid carbolic and cresylic acids. The comparative antiseptic power of soaps may be tested by adding equal weights, in solution, to equal weights of flour paste, and, after exposing these to the air under identical conditions, noting the day on which mould first appears on each. The so-called "coal-tar" soap, or "sapo carbonis detergens" owes its disinfecting properties to a small quantity of carbolic acid in the coal tar.
It may be assumed as a pretty safe working rule, that almost any desired substance, whether soluble in water or not, can be mixed in this manner with soap, provided that (1) it is not too volatile at the lowest temperature at which the soap can be manipulated;
(2) that if insoluble in water or alcohol it is in a state of very fine subdivision;
Thus Cleaver incorporates terebene, copper alum, or iron alum; Robottom adds borax or other borates; and others select camphoraceous products obtained by artificial oxidation of turpentine oil, such as "sanitas."