Inasmuch as a soap is the alkali salt of a fatty acid, the oil or fat from which soap is made must have as a constituent part, these fatty acids. Hydrocarbon oils or paraffines, included in the term "oil," are thus useless in the process of soap-making, as far as entering into chemical combination with the caustic alkalis is concerned. The oils and fats which form soap are those which are a combination of fatty acids and glycerine, the glycerine being obtained as a by-product to the soap-making industry. NATURE OF A FAT OR OIL USED IN SOAP MANUFACTURE. Glycerine, being a trihydric alcohol, has three atoms of hydrogen which are replaceable by three univalent radicals of the higher members of the fatty acids, _e. g._, OH OR C_{3} H_{5} OH + 3 ROH = C_{3} H_{5} OR + 3 H_{2}O OH OR Glycerine plus 3 Fatty Alcohols equals Fat or Oil plus 3 Water. Thus three fatty acid radicals combine with one glycerine to form a true neutral oil or fat which are called triglycerides. The fatty acids which most commonly enter into combination of fats and oils are lauric, myristic, palmitic, stearic and oleic acids and form the neutral oils or triglycerides derived from these, _e. g._, stearin, palmatin, olein. Mono and diglycerides are also present in fats. SAPONIFICATION DEFINED. When a fat or oil enters into chemical combination with one of the caustic hydrates in the presence of water, the process is called "saponification" and the new compounds formed are soap and glycerine, thus: OR OH C_{3}H_{5} OR + 3 NaOH = C_{3}H_{5} OH + 3 NaOR OR OH Fat or Oil plus 3 Sodium Hydrate equals Glycerine plus 3 Soap. It is by this reaction almost all of the soap used today is made. There are also other means of saponification, as, the hydrolysis of an oil or fat by the action of hydrochloric or sulfuric acid, by autoclave and by ferments or enzymes. By these latter processes the fatty acids and glycerine are obtained directly, no soap being formed. FATS AND OILS USED IN SOAP MANUFACTURE. The various and most important oils and fats used in the manufacture of soap are, tallow, cocoanut oil, palm oil, olive oil, poppy oil, sesame oil, soya bean oil, cotton-seed oil, corn oil and the various greases. Besides these the fatty acids, stearic, red oil (oleic acid) are more or less extensively used. These oils, fats and fatty acids, while they vary from time to time and to some extent as to their color, odor and consistency, can readily be distinguished by various physical and chemical constants.