31. Fats. One other constituent shown in the chart remains to be mentioned. This is the fat or fixed oil, called "fixed" because, unlike the "volatile" oils, it does not evaporate at ordinary temperatures. A little of this oil may be separated for examination by soaking "whole wheat" flour, bran, or corn meal in naphtha, and then pouring off the latter into a shallow dish. The naphtha will evaporate, leaving behind the oil which it had dissolved.

In chemical composition the fats agree with the carbohydrates in consisting of carbon, hydrogen, and oxygen; the difference being that there is always less than half as much oxygen as hydrogen. Like the carbohydrates their use as food is for yielding warmth and strength, and they may make the body fatter; but as in these respects fats are more than twice as effective as carbohydrates their importance in the various grains is much greater than would appear from the comparatively small amounts which are present. This fact enables us to understand the great value of maize, for example, in fattening animals.

With foods rich in oil there is this drawback, however, that after a limited time they are apt to spoil with keeping, while starchy foods remain practically unchanged as long as they are dry. Thus wheat, which contains less oil than maize, keeps better, and its deficiency in this ingredient we fully make up for by eating butter on our bread.