Solidified Glycerine.—The preparation of this by Price's Candle Co. is a trade secret, but Morflt recommends the following method. Heat to 154i° (310° F.) a mixture of 350 lb. hot-pressed fs.tty acids, 150 lb. white oleic acid, 200 lb. best rosin. To this, add 135 lb. Jarrow 52° ash (p. 1771) in 25 gal. boiling water. When the soap-paste is quite homogeneous, which should be in about an hour, add 250 lb. pure glycerine, and stir well. If a sample be not transparent when cold, add glycerine until this is the case, controlling the amount of glycerine by testing 2-1b. 'samples of the soap with glycerine over a gas flame. This soap has the following composition :—Fatty acids, 34.0 ; rosin, 13.0; soda, 4.6; water, 15.4 ; glycerine, 33.0.
Since it is beyond the scope of this article to devote more space to the detail of this part of the subject, it may be mentioned briefly that the various shaving-soaps and creams are wholly or in great part potash-soaps ; that soap-essences are usually alcoholic solutions of soft-soap; that opodeldoc is a solution of soap in enough alcohol to make a jelly when cold ; that "floating " soaps are made by dissolving soaps in a small quantity of water, and agitating the solution violently in contact with air ; and that powdered soaps are made from any pure soap, cut into shavings, thoroughly dried, and then ground to fine powder and sifted. It may also be well to call attention to the fact that nearly all the so-called washing-powders, soap-powders, and essences of soap, frequently coniain no soap at all, and are merely mixtures of soda-ash, common salt, and sulphate of soda, with occasionally a trace of dry powdered soap.
Theory of the Action of Soap : its Valuation and Analysis.—The mode in which soap facilitates the removal of dirt is by no means clearly understood, and probably depends upon a variety of causes, partly physical, partly chemical. Unquestionably much of its power is due to the alkali it contains, which unites with and renders soluble the grease that forms so large a portion of much of OUT dirt ; but it can hardly be true, as is maintained by Borne, that the value of a soap depends solely upon its percentage of alkali, since, if that were so, solutions of silicate, carbonate, or aluminate, of soda, containing the same percentage of soda as soap, ought t,o do as much work, which is notoriously not the case. Further, since the proportion of alkali in a soap is inversely as the equivalent weight of its fatty acids, those soaps with fatty acids of the smallest equivalent weights (e.g. coco nut-oil) ought to be the most advantageous. Geiger, who advocates this view, givee the following table of anhydrous soaps.
Cold water never is in contact with an alkaline stearate or oleate (the soap of commerce there fore) without decomposing it; the neutral salt is resolved into alkali, which dissolves, and an acid salt, which is precipitated as insoluble. Hence soap even in the purest cold water produces
turbidity, although, when treated with warm water, it dissolves entirely. Again, since every kind of soap, when it lee,ves the copper. is more or less concentrated solution of anhydrous soap in water, when cold and firm, it also is subject to the BilDle decomposition; this is the cause of the slender silky crystalline fibres set in a semi-transparent matrix, so often seen especially in "neat " soaps.
When soap is rubbed in use against the surface to be cleansed, it is obvious that its greater or less hardness is an important consideration, since a harder soap requires much labour to detach enough, while a softer soap wastes away rapidly. It has been already shown that, eeteris parlous, the hardness of a soap depends upon how mueh potash it contains; but where soda only is the base, the question of the comparative solubilities of the soda salts of the fatty acids has to bo considered. While oleate of soda is freely soluble in 10 parts of water, stearatc of soda is scarcely affected thereby ; or in other words, the salts of oleic acid ere far more soluble than those of stearic. Hence the hardness of a soap depends, not merely upon the base used, but upon the relative quantities of steario and oleic acids in its composition; this point will be again referred to in the a.nalysis of soaps.
The impurity of the water employed with soap has a material influence upon its consumption. Rain-water, and next to it, river or lake-water, is the best, while spring-water should be avoided if possible ; suoli water is more or less " hard," owing to the presence iu it of salts of lime (chiefly the carbonate and sulphate), some of which may be removed by boiling, or, more completely, by the addition of carbonate of soda. When a soluble soda-soap comes into contact with lime salts in solution, mutual decomposition occurs, resulting in the formation of insoluble limc-soaps, which have no deteigent action. Until all the lime has been thus removed, the soda-soap refuses to cleanse, and hence much of it is wasted. It is obvions also that the presence of any acid in the water, or ou ths surfaces to be cleansed, will decompose soap, uniting with its alkali, and destroying ita detergent power. When nothing but hard water can be prochred, or when much grease has to be removed, no soap will be found so economical as the old-fashioned eurd-mottled, entangled in the interstices of which are appreciable quantities of caustic soda leys.