To revert for a moment to the combination of sulphnrie acid and soda,—considerable heat is evolved in the process, which has its parallel in aaponification; further, if either constituent had been in excess, there would have resulted a mixture of neutral sulphate of soda with the remainder of whichever constituent was in excess. lf, however, twice 49, or 98, parts of sulphurie acid had been taken, an acid sulphate, or bisulphate, of soda, would have been formed. The fatty acids are remarkable in this respect, that their combining proportions or eqnivalents are much higher than those of mineral acids. Thus to combine with 31 parts of soda (100 per cent.), 284 parts of stearic acid are required, 282 parte of oleic acid, 256 parts of palmitic acid, and only 200 parts of laurie acid, one of the chief fatty aeids of coco-nut-oil. Hance, while tallow gives the least " yield" of soap, among the fats usually employed,coco-nut-oil givea the most, and palm-oil occupies au intormediate position.
What has been said above with regard to 31 parts of soda, applies with equal force to potash, replaciug 31 by 47, which is the combining proportion of potash.
From the explanation now given, it is easy to calculate the quantity of soda or potash necessary to oompletely saponify,— leo of Soda, 100 per cent. Potash, 100 p•r cent.
Tallow .. 10.50 lb. 15.92 lb.
Palm-oil 11.00 „ 16.67 „ Coco-nut-oil 12.44 „ 18.86 „ Oloio aeid 10.52 „ 15.95 „ It ie obvious that the proportion of alkaline ley to be used must be regulated by its strength ; thus, if ley eontaining 20.0 per oent. soda be used, i.e. its weight of soda at 100 per cent., five times the given weights must be used for 100 lb. fatty material.
It will be convenient here to explain briefly the causes of the differences in combining propor tion among the various fats. Their fatty acids are all composed of the three elements, carbon, hydrogen, and oxygen, combined in different proportions of thetwo first-named, hut all containing the samo quantity of oxygen. They may be arranged in a series, known 118 the "adipic," of which the lowest term, formio acid, contains 12 parts by weight of carbon, 2 parts-hydrogen, and 32 parte oxygen. The other terms of the series differ from eaeh other by 12 parts carbon and 2 parts hydrogen. The following table gives some of the prineipal terms of this series, 0„112„.0„ from whioh, it is olear that the differenoes in their equivalents are due to the differences in the quantities of carbon and hydrogen entering into their composition, which also affect their 'melting and boiling-points.
The fatty acid of the fluid constituent of most natural fats and oils, called oleic acid, belongs to another series, Calsa_202, known as the " acrylic ;" its formula is 01811340w and its equivalent is 282.
For reasons which are not within the scope of the present article, the true chemical equiva lent of these fatty acids is always taken at thrice the above numbers. The acids can all form salts containing once, twice, or thrice 31 parte of eoda, and it requires three of the above equivalents of auy acid to form a neutral fat with one equivalent of glycerine, so that the process of the saponifica tion of a neutral fat by caustic soda may be thus represented,— (3 equivalents, C3H,(C„H3502), + 3NaHO = 3(C„1.1.02Na) + Ca11,(OH), (1 equivalent, stearic acid Tristearine. Caustic soda. Sodium stearate Glycerine. glycerine.) or soap.
and soda.) 890 + 120 = 918 + 92 Consult alao Candles, pp. 579-80. Glycerine will be treated of separately at the end of this article (pp. 1798-1802).
RAW MATETUALS.—The raw materials employed in the production of pure soaps are (1) fatty matters of various kinds, (2) rosin, or colopbony, (3) solution of caustic soda or potash. In the article Oils and Fatty Subatances, will be found fall descriptions of the different kinds of fats and oils suitable for soap-making ; on pp. 1458-62, are directions for refining and bleaching them, pre paratory to their saponification ; and on pp, 1462-77, are instructions for their analysis, the detec tion of adulterants, &e. Rosin will be discussed under Resinous and Gummy Substances (see also Wax), and Alkali contains a full account of the manufacture of soda and potash. But very few general remarks, therefore, are necessary in treating of raw materials, and these will be especially practical in their character.
Since the selection of the raw material depends entirely upon the kind of soap which it is desired to produce, it may be remarked at the outset, that soaps axe divided into two classes, " hard " and " soft." For the production of what is technically called " soft soap," it is necessary to use potash as an alkali, to the almost entire exclusion of soda. Potash soaps are deliquescent, and do not dry up when exposed in solution to the air, but retain water enough to form a soft slimy jelly. Soda soaps neitber retain so much water, nor absorb it from the air, but harden when exposed, partly from loss of water. This difference is best seen in thc case of the salts of pure fatty acids thus : — 100 parts dry potassium oleate absorb 162 parts water from the air.