Soap.—­Soap is a salt in the chemical sense, and this leads to a wider definition of the term “salt” or “saline” compound.  Fats and oils, from which soaps are manufactured, are a kind of quasi salts, composed of a fatty acid and a chemical constant, if I may use the term, in the shape of base, namely, glycerin.  When these fats and oils, often called glycerides, are heated with alkali, soda, a true salt of the fatty acid and soda is formed, and this is the soap, whilst the glycerin remains behind in the “spent soap lye.”  Now glycerin is soluble in water containing dissolved salt (brine), whilst soap is insoluble, though soluble in pure water.  The mixture of soap and glycerin produced from the fat and soda is therefore treated with brine, a process called “cutting the soap.”  The soap separates out in the solid form as a curdy mass, which can be easily separated.  Certain soaps are able to absorb a large quantity of water, and yet appear quite solid, and in purchasing large quantities of soap it is necessary, therefore, to determine the amount of water present.  This can be easily done by weighing out ten or twenty grams of the soap, cut in small pieces, into a porcelain dish and heating over a gas flame, whilst keeping the soap continually stirred, until a glass held over the dish no longer becomes blurred by escaping steam.  After cooling, the dry soap is weighed, and the loss of weight represents the amount of moisture.  I have known cases where soap containing about 83 per cent. of water has been sold at the full market price.  Some soaps also contain more alkali than is actually combined with the fatty acids of the soap, and that excess alkali is injurious in washing silks and scouring wool, and is also not good for the skin.  The presence of this free or excess alkali can be at once detected by rubbing a little phenolphthalein solution on to the freshly-cut surface of a piece of soap; if free alkali be present, a red colour will be produced.

LECTURE VII

SHELLAC, WOOD SPIRIT, AND THE STIFFENING AND PROOFING PROCESS

Shellac.—­The resin tribe, of which shellac is a member, comprises vegetable products of a certain degree of similarity.  They are mostly solid, glassy-looking substances insoluble in water, but soluble in alcohol and wood spirit.  In many cases the alcoholic solutions show an acid reaction.  The resins are partly soluble in alkalis, with formation of a kind of alkali salts which we may call resin-soaps.

Shellac is obtained from the resinous incrustation produced on the bark of the twigs and branches of various tropical trees by the puncture of the female “lac insect” (Taccardia lacca).  The lac is removed from the twigs by “beating” in water; the woody matter floats to the surface, and the resin sinks to the bottom, and when removed forms what is known as “seed-lac.”  Formerly, the solution, which contains the colouring matter