SOAP.—Properties. 769

In all saponifiable fats the same reaction ensues,though the soap produced contains different acids,according to the kind of grease or fat employed.Thus, in the case of palm oil, the soap consists forthe most part of palmitate of sodium or potassium.When spermaceti is used, the soap produced con-sists of cetylate of potassium or sodium ; and insteadof glycerin, an analogous body termed ethal isfound in the mother liquor. By far the greatermajority of fats, however, yield when treated withalkalies soaps containing only stearate, palmitate,and oleate of potassium, if a soft soap, and of sodiumin the case of hard soaps. Rosin, though employedin the manufacture of soap in the place of fixed oilor fat, is not to be considered analogous to a fat inits chemical constitution, or to be capable of formingwith an alkali a proper soap by itself. Rosin is anindefinite mixture of three distinct bodies possessingwell - marked acid properties; and hence namedsylvic, pinic, and colophonic acids, each of which iscapable of entering into combination with alkalies,and of readily dissolving in alkaline solutions, thoughtotally insoluble in pure water.

For the complete saponification of fats or oils anexcess of alkali is requisite; this is the more willinglyemployed, that it can be removed again withoutdifficulty.

The production of soap is by no means a momen-tary process. On the contrary, the manufactureconsists of a number of stages, and these occupya considerable length of time from the first mix-ing of the fat with the alkali, when an emulsion isproduced, to the formation of soap ready for use,or even to that point when the whole of the alkaliis saturated with fatty acid. Acid salts are firstproduced with the oily acids, and these hold the re-mainder of the fat in solution, or in a state of division,until it also is able to combine with alkali, andtransform the acid into neutral salts or into finishedsoap. This reaction may easily be observed, if thefat is boiled with one-half the required quantity ofalkali; the whole of the oil is in such case dissolved,but the solution becomes turbid on cooling, andwhen water is added, and the mixture again boiled,unsaponified fat separates, showing that this hadonly been retained in the fluid by the stearate ormargarate of the alkali that had been formed.

Hard and Soft Soap. —Soaps are divided into twoclasses, differing from each other in the material fromwhich they are produced, and are distinguished bythe names hard and soft soap. Hard soaps are madefrom fats, or vegetable fat oils, and soda; for softsoaps, the oleaginous material is usually a fish oilor vegetable drying oil, and the alkali, potash.Hard soaps may, however, be made with potash,provided a solid fat be employed ; but soda soapsare invariably harder than potash soaps, the unctuousmaterial being the same. The hard soaps of animalfats are either white or marbled, and are particularlyapplicable for washing the finer articles of dress,the soft soaps being more caustic than the solid sodasoaps. Of the hard soaps of commerce those madewith oils, with the exception of palm oil, consistVOL. II.

of oleate, with only a small proportion of stearateof sodium; those made with animal fats are mix-tures of stearate, palmitate, and oleate of sodium.On the Continent , and wherever wood is abundant,potash is frequently introduced in admixture withsoda in the production of hard soaps.

Properties of Soaps . —Potash and soda soaps arereadily soluble in hot water and alcohol; both solu-tions solidify to a jelly if concentrated. Opodeldocis this jellied soap mixed with alcohol. The additionof a quantity of cold water to the aqueous solutionproduces a precipitation, the neutral salts of stearicand palmitic acids decomposing into free alkali, whichremains in solution, and acid stearate and palmitateof alkali, which precipitates in the form of pearly,crystalline scales. Potash soaps are more solublein water than those containing soda. Stearate ofsodium may be regarded as the type of hard soaps ;in contact with ten times as much water, it suffersno striking change, while stearate of potassiumunder the same circumstances forms a thick pasteor a viscid solution. In these respects the palmitatesof potassium and sodium closely resemble the cor-responding stearates. Oleate of sodium is solublein 10 parts of water, and oleate of potassium in 4parts. The latter forms a jelly with 2 parts, andpossesses such a strong affinity for water, that 100parts absorb 162 when exposed to a humid atmos-phere. It follows, therefore, that soaps are soft inproportion to the oleates, and hard in proportionto the stearates and palmitates they contain, sothat the softness or hardness of soap is not solelydependent upon the base that is employed.

Soap of Commerce —that is, stearate, palmitate, oroleate of alkali—is never dissolved by cold waterwithout decomposition. The neutral salts are re-solved into alkali, which dissolves, and into an acidsalt, which is precipitated. The same decompositionoccurs when hot solutions of soap—particularly ofdilute—are cooled.

The occurrence of these decompositions, whichensue equally with the palmitates and oleates ofpotassium and sodium, as with the stearates, affordsan easy explanation of the appearance of the whitishturbidity always produced when soap is dissolved ineven the purest water, and show, at the same time,that the alkalinity of soap-suds is solely due to theliberation of caustic potash or soda; and this it isthat affords the possibility of removing fatty im-purities in water, which is the sole object of washingwith soap.

It is of the utmost importance to the soap-boilerthat he be well acquainted with the physical com-portment of soaps with solutions of different salts,as, for instance, with chloride of sodium, carbonateof soda, the corresponding potassium salts, chlorideof ammonium, and various others. These, althoughnot absolutely essential to the formation of soaps,are very much concerned in the separation of theforeign matters that render hard soap impure; andare also useful to impart to it the proper amount ofwater. In practice, a solution of chloride of sodiumis always employed for this purpose. The peculiar97