SOAP, RAILWAY-GREASE, AND GLYCERINE.
Soap (Fn., Savon ; GER., Seife).—Although soap has been made and used for many centuries (it is mentioned by Pliny, and the remains of a soap-factory, with lime, ite., remaining in it, are still shown in Pompeii), the principles which should guide its manufacture have only beea discovered in quite recent times. A proper comprehension of these principles is indispensable to every oue who would become a successful manufacturer, because soap-making is essentially a chemical operation ; but as they eau only be dealt with very briefly here, the reader unacquainted with them is recommended to consult any modern elementary work on chemistry.
Although, in ordinary parlance, the term " soap" denotes simply that combination of fatty matter with alkali which, hy its detergent properties, aids in the removal of grease and dirt in washing (in which sense alone will it be used in this article), it is highly important to remember that " soaps" as a class are, strictly speaking, " salts," using that term in the chemical sense. Every salt contains an acid and a base, having opposite properties, and producing by their union a third substance differing from either. Thus Glauber's salts, sulphate of soda, or eodium sulphate (all are synonymous terms), is a compound of sulphuric acid and soda, or Salt = acid + base.
Sulphate of soda = sulphuric acid -I- soda.
All the neutral fats of commerce which are used in soap-making, such as tallow, palm-oil, coco nut-oil, cotton-seed-oil, and greases of various kinds, are also, from a chemical point of view, " salts" of which the " base " is (not trod.a but) glycerine, and the "acid " (not sulphuric but) a mixture of various " fatty acids," which by proper means, may, if desired, be separated from each other, and prepared iu a state of greater or less purity. Hence,— Salt acid + base.
Neutral fat (e. g. tallow) = various fatty acids 4- glycerine.
Theoretically, soap-making is nothing more than turning out the glycerine base by a strong mineral base, or alkali, such as potash or soda. Hence,— Neutral fat -I- Alkali = { Fatty acids + Alkali, or " Soap " + Glycerine.
As, however, certain oils much used by the soap-maker are already fatty acids, and contain no glycerine (e. g. oleic acid, sometimes erroneously called oleine, a bye-product of the candle-factory),
the formation of. soap from them is simply a direct combination of fatty acids with the proper pro portion of alkali. This process will be dealt with in describing special soaps.
While, therefore, chemically speaking, any combination of fatty acids with a mineral base is a " soap," in practice no soaps are made except with potash or soda, as only those soaps are soluble in water ; all others, such as those formed by the union of fatty acids with lime, baryta, or even with the oxides of the metals, as lead, copper, Sze., are insoluble in water, though some of them are used in pharmacy : a " plaster," for instance, is usually a soap from the fatty acids of soft oils, with oxide of lead as a base, and chemically speaking, is an oleate of lead.
A further acquaintance with the theory of salts will make it clear that, in the case of mineral acids and bases there is a certain definite proportion peculiar to each, in which (or in a simple multiple of which) they combine with each other. Thus 31 parts of pure caustic soda (100 per cent.) require 49 parts of oil of vitriol (100 per cent. sulphuric acid) to form the neutral salt, sulphate of soda, and in the operation, 9 parts of water are formed, the hydrogen of which is derived from the acid, and the oxygen from the base. The combined weight of the products is of courze exactly equal to the sum of the weight of the constituents, or,— 31 parts soda + 49 parts sulphuric acid = 71 parts sulphate of soda -I- 9 parts water.
This number, called the " combining pmportion" or " equivalent " of each substance, is determined by ohemical research. It can scarcely be too strongly insisted on that the fatty acids have their equivalents also; thus the determination of the quantity of soda necessary for their saponification is a matter of calculation, and home the varying equivalents of the different fatty acids is the real explanation of the well-known fact that the " yield " of soap is so different from various fats.