All fats and vegetable oils can be resolved into their two constituents, the acid and the glycerine. This can be brought about by heating the fat with water to about 200° C. This operation must be carried out in a vessel capable of withstanding pressure and closed with a safety valve; otherwise, the requisite temperature could not be obtained. After this treatment, there is left in the vessel an oily layer which solidifies on cooling and an aqueous layer which contains the glycerine. The solidified oily layer is the fatty acid. In the case of mutton or beef tallow, it would be mainly a mixture of stearic and palmitic acids. This mixture is used to make “stearin” candles. The acids themselves are wax-like solids without any distinctive taste. Stearic acid melts at 69° C. and palmitic at 62° C. They have no perceptible action on the colour of litmus, neither have they any solvent action on metals or carbonates. We should not recognize these substances as acids at all were it not for the fact that they combine with alkalis, forming salts. The salts of the fatty acids are called soaps. To make soap, the fat is boiled with caustic alkali or caustic lye, as it is more often called. This breaks the fat up primarily into the acid and glycerine; but in this case, instead of obtaining the acid as the final product as we did above by heating with water under pressure, we get the sodium or potassium salt of the acid according to the alkali used. When caustic soda is used, the product is a hard soap; when caustic potash is used, it is a soft soap. The treatment of fats in this way with caustic alkalis is called “saponification.” CHAPTER VIII MILD ALKALI Caustic and Mild. There are two classes of alkalis distinguished by the terms caustic and mild. If a piece of all-wool material is boiled with a solution of caustic soda or potash, it dissolves completely, giving a yellow solution. Mild alkali will not dissolve flannel, though it may have some slight chemical action causing shrinkage. Partly for this reason, and partly because commercial washing soda often contains a little caustic soda, woollen garments must not be boiled or even washed in hot soda water. The disintegrating action of the caustic alkalis is also illustrated by the use of caustic soda in the preparation of wood pulp for paper making. Tree trunks are first torn up and shredded by machinery; but notwithstanding the power of modern machinery, the fibre is not nearly fine enough for the purpose until it has been “beaten” with a solution of caustic soda, whereby the pulp is brought to a smooth and uniform consistency like that of thin cream.