free acid present in the compound, that is, an acid which is present uncombined with the glycerine. This free acid is usually present in small quantities and is more abundant in the overripe and older plants than in the freshly matured parts. The natural oil also contains certain other ingredients which may be regarded as impurities, and which it is necessary to remove from the oils by a process of purification or refining before they are ready for the table. These impurities may be of a mechanical nature, that is, consisting of parts of the material itself from which the oil is expressed or of certain juices not oils which are found in the animal tissue, portions of protein and other forms of nitrogenous matter, and traces of carbohydrates and gums. The oils have certain definite chemical reactions which are common to them as a class. Among these may be cited, principally, the faculty of absorbing, under certain conditions, the halogens, namely iodin, bromin, and chlorin.
Without entering into any technical description of this process it is sufficient to say here that the degree of absorption of iodin is in a measure the test for the varieties of oil. The different vegetable oils have, as a rule, certain definite relations to the absorption of iodin by means of which they may be to a certain extent identified or separated from similar bodies. The degree of absorption is expressed in the percentage by weight of the oil itself and is known as the iodin number. If, for instance, a gram of any particular oil absorbs one gram of iodin, it is said to have an iodin number of 100. Many oils absorb more than their own weight of iodin, while many others absorb very much less. Another characteristic of oil is found in the fact that with certain reagents, such as an acid either in a dilute state or in a concentrated state, definite colors are produced which are characteristic of the variety of oil in question. As an example of this may be cited the faculty which cottonseed oil has of reducing nitrate of silver to the metallic state, leaving the silver in that finely divided form which has a black color. This is the only oil in common use which has this faculty, and hence it may be regarded as a characteristic test.
Another characteristic chemical property of cottonseed oil is the color which is produced in the Halphen reaction, which has already been described.
One of the most valuable chemical properties of oil is the amount of heat which is produced when it is burned. Inasmuch as oils in relation to their food value are useful chiefly for the production of animal heat, this chemical property becomes of great hygienic and dietetic significance. Of all classes of food products the oils and fats have the highest calorific power. If, for instance, it is said in general that one gram of carbohydrates, such as sugar or starch, on complete combustion will yield 4,000 calories, one gram of protein 5,500 calories, then one gram of oil or fat will yield 9,300 calories. The fats and oils vary among themselves in respect of the number of calories yielded, but all of them give, approximately, the number last mentioned. It therefore
