SOAP. A name given to those bodies which are compounds of the alkalies with fat and the filed oils. The earths, and the other metallic oxides also, com bine with fat and oils, forming neutral compounds. The former have been called earthy, and the latter metallic soaps. The soaps formed by the alkalies bare the distinguishing character of being soluble in water and alcohol. The earthy soaps are perfectly insoluble : and since any of the earths have a stronger attrac tion for oil than the alkalies, the alkaline soaps are always decomposed by the earths. This occasions the curdy appearance when soap is used with water con taining any earthy or metallic salt: it is from this quality that waters are said to be hard. Soap was imperfectly known to the ancients. It is mentioned by Pliny as made of fat and ashes, and as an invention of the Gauls. Aretreus and others inform us, that the Greeks obtained their knowledge of its medical use from the Romans. Its virtues, according to Bergius, are detergent, resolvent, and aperient; and its use recommended in jaundice, gout, calculous complaints, and in obstructions of the viscera. Many have boasted of its good effects in urinary calculous affections, especially when dissolved in lime water, by which its efficacy is considerably increased; fur it thus becomes a powerful solvent of mucus, which an ingenious modern author supposes to be the chief agent in the formation of calculi; it is, however, only in the incipient state of the disease that these remedies promise effectual benefit, though they generally abate the more violent symptoms where they cannot remove the cause. With Boerhaave, soap was a general medicine • for as he attributed most complaints to viscidity of the fluids, he, and most of Boerhaavian school, prescribed it in conjunc tion with different resinous and other substances, in gout, rheumatism, and various visceral complaints. Soap is also externally employed as a resolvent, and gives name to several officinal preparations.
The soaps used in the manufactures and domestic economy, are made with the fixed alkalies, combined with different kinds of fat and oil. These, in the manu• facture of soap, are divided into two principal varieties, viz. hard and soft. The alkali employed for hard soap is soda, generally obtained from the different sea vegetables, and called by different names, according to the name of the plant, in different countries. Most of the alga, but particularly the fucus and salsola, afford soda by burning. The vegetables are first dried, and then burnt in pits formed with loose stones. The earthy matter, and the soda, with some neutral salts, fuse into a crude mass, in which state it is sold. This substance is fur nished in great abundance from the Highlands of Scotland, under the name of kelp, and from Alicant, in Spain, under the name of barilla. In France it is known by the name of varec ; this being the name of the plant from which it is generally produced there. It is oommonly, however, in this state that it comes to the soap-maker, varying frequently in its value, and often occasioning much uncertainty in its employment. It should be the first business, therefore, of the manufacturer, to assay the substance from which he gets his alkali, even before he purchases it. When the exact value of the alkali is known, it is then to be treated as follows, to prepare it for mixing with the fat. The kelp, or barilla, is first to be pounded, and then mixed with one-fifth its weight of quick lime, in a large vat. These vats aregenerally three or four in number to each boiler. Besides these vats for the infusion of .crude alkali, each of them has a cavity made under it. The bottom of each vat is even with the ground, the under cavity being sunk below, and is intended to receive the liquor which runs from a plug-bole in the upper vat, when the infusion has gone on to a certain extent. One of these vats, with its under reservoir, is sufficient for one boiling, but they are generally all at work, in order to give time for the solution of the alkali from the crude mass. In charging a vat, the barilla, kelp, or potash, and sometimes mixtures of these, are first coarsely powdered and mixed with quick lime, also coarsely powdered; some water is then thrown upon these, to slake the lime. In the side of the vat some straw is first placed about the plug-hole, to prevent bits from passing through. The vat is now charged, and water poured upon the materials till it stands considerably above the solid mass ; after stand ing several hours the plug is withdrawn, to let out the solution into the lower reservoir. The plug is now returned, and fresh water poured upon the mate
rials. Some, or all of the first ley is now removed into one of the other lower reservoirs before the second infusion is drawn off. This is done that the soap boiler may always have at command two legs of different degrees of strength, as, in the course of every boiling, he finds it necessary to use sometimes the weak, and, at other times, the strong. The number of waters to be added to the materials, depends upon the judgment of the workman, who, by his taste, can tell when the water has dissolved the whole of the alkali. The ley being ready to lade out of the reservoir, which is near to the boiler, the tallow or oil, first weighed, is put in. When it is sufficiently melted, the workman begins by adding the ley and stirring the mixture. The alkali and the oil soon begin to unite, forming a milky fluid. As more ley is added, and the stirring continued, the liquid thickens. This is continued generally for thirty hours, and frequently more, till small portions of the soap, taken out from time to time, assume a proper consistence, which the workman, by constant experience, understands. He now adds a quantity of common salt, which has the effect of separating the watery part from the soap, which contains a portion of neutral salts, that existed in the crude alkali, especially when more than enough has been added. The fire has now to be withdrawn, and the mass left to cool. The watery part will be found at the bottom, and requires to be drawn out by a pump, which is a fixture on the side of the boiler. When this has been removed the fire is re kindled, and if the mass does not melt freely, a little water is added. As soon as tho whole becomes liquid, and is made uniform by agitation with wooden poles, the fire is again withdrawn, and the mass allowed to assume a proper consistence for lading. It is laded into square moulds; these are composed of a number of strata lying one upon another, so that when the soap has become solid, each layer of frame-work can be removed, beginning at the top, and the soap is cut into cakes with a small piece of brass wire at every interval ; these cakes are afterwards cut into square prismatic pieces, in which state they are sold. Yellow hard soap is formed of similar proportions of soda and tallow with the last; but it also contains rosin, and sometimes palm oiL In boiling the yellow soap, the rosin, oil, and tallow, are put into the boiler first. The ley is prepared in a similar vat, and managed, in other respects, like the white soap.
Soft soap differs in its composition from hard, in containing no alkali, but potash. Soft soap made with colourless fat, such as tallow, is a white unctuous substance, about the consistency of lard. If the fat be coloured, the soap par takes of the same. In France, and other parts of the continent, it is generally coloured, sometimes with metallic oxides. Those made with yellow oil are some times coloured with indigo, which gives them a green colour. The oils employed' are seldom olive oil, but the cheaper oils, such as rape•oil, the oil of hempseed, linseed, and others. In Holland it was made with whale-oil. This oil was forbidden on some parts of the continent, on account of its disagreeable smell. In this country, however, all the soft soaps are made with whale-oil, which gives a transparent mass of a yellow colour. In commerce, however, we do not find it uniform in its colour; besides the yellow part, it appears interspersed with white spots, giving the whole a strong resemblance to the inside of a dried fig.
Soap is chiefly used in washing and whitening linens, c eansing woollen cloths from oil, whitening silk, and freeing it from the resinous varnish with which it is usually covered, and for various purposes by the dyers, perfumers, fullers, &c. The alkaline lixiviums being capable of dissolving oils more effec tually than soap, might be employed for the same purposes ; but when this activity is not mitigated by oil, as it is in soap, they are capable of altering, and even destroying entirely, by their causticity, most substances, especially animal matters, as silk, wool, and others ; whereas soap cleanses from oil, almost as effectually as pure alkali, without danger of altering or destroying, which renders it peculiarly useful.