OILS AND FATTY SUBSTANCES (FR., Huiles et Corps Gras; GER., Ode and Felt waaren).
The dual heading " oils and fatty substances" is rendered necessary by the fact that, in everyday language, the specific terms " oil " and " fat " are very widely, and often erroneously, applied. The name " oil " is made to embrace three distinct classes of bodies :—(a)" fixed" or "fatty" oils, (b) and " essential " oils, and (c) and other "mineral" oils.
The first class comprises a number of organic bodies, composed of carbon, hydrogen, and a little oxygen, viscid liquids, communicating a permanent stain to paper, insoluble in water, and, as they occur in nature, mostly mixtures of different simple fats, which, by saponification, are resolved into fatty acids and glycerine. For further information upon the subject of saponification, the reader is referred to the articles on Candles and Soap ; the identification of the fatty acids is discussed under the section of the present article entitled Detection and Analysis. The term " fat " is applied to these oils when they are in a solid state ; thus the same product may be an " oil " in one climate, and a " fat " in another.
The second class, volatile and essential oils, consist eitber wholly of carbon and hydrogen, or of these elements supplemented by less proportions of oxygen, nitrogen, and sulphur. They have a thin, oily consistence, volatilize completely at high temperatures, possess powerful and peculiar odour and flavour, are very inflammable, and sparingly soluble in water. Many of them occur ready formed in organic bodies, chiefly of the vegetable kingdom, and are then true essential oils ; others, which are volatile but not essential, are produoed by dry distillation, fermentation, and other changes.
The third class, mineral oils, belong strictly to the preceding, being true volatile oils ; but their immediate sources, preparation, and application, differ so widely, and they form, moreover, such an important branch, that it will be best to consider them separately. Neither essential nor mineral oils can be spoken of as " fatty substances."