19. CASEIN is the curdy ingredient of milk, and a highly important food-substance. Its coagulation in milk takes place not from heat, but by the addition of an acid, and also when milk becomes sour from exposure to the air. It is commonly effected, however, by introducing a piece of _rennet_, a preparation made from a calf's stomach. The _curds_, or casein, may then be separated from the _whey_, {59} and made into cheese, by pressing it sufficiently to drive off the water. [Sidenote: 20. What are the fats? The oils? How supplied? How alike? Emulsifying? Example? How do we know it?] 20. THE FATS OR OILS.--This is the second group of organic foods. Those which are more solid are called _fats_: the more fluid ones are the _oils_. Oleaginous substances are supplied in both animal and vegetable food; but, from whatever source derived, they are chemically much alike. They are insoluble in water, and yet they unite readily with the watery fluids of the body, and are by them conveyed to its various parts for their nourishment. This is due to their property of "emulsifying;" that is, they are held in suspension, in a finely divided state, in water. Ordinary milk is an example of an _emulsion_. We know that it contains fat; for butter is obtained from it, and, under the microscope, the minute oil-globules may be distinctly seen. [Sidenote: 21. Whence are fatty articles of food derived?] 21. In our country and climate, and also in colder climates, fatty articles of food are principally derived from the animal creation, such as meat or flesh, milk and butter. But most of the bread-stuffs contain more or less fat or oil; Indian meal as much as nine parts in a hundred. [Sidenote: 22. Appetite of persons in cold climates? What do they require? Upon what must they rely? Why? The Esquimaux? Laplander? Olive and palm?] 22. Among persons living in cold climates, the appetite for oleaginous food is especially eager; and they require large quantities of it to enable them to resist the depressing influences of cold. Since vegetation is scanty and innutritious, and the waters of the frozen regions abound in animal life, they must rely wholly upon a diet derived from the latter source. The Esquimaux consumes daily from ten to fifteen pounds of meat or blubber, a large proportion of which is fat. The Laplander will drink train-oil, and regards tallow-candles as a great delicacy. In hot climates, on the contrary, where flourish the olive {60} and the palm, this kind of food may be obtained from vegetable sources in abundant quantities. [Sidenote: 23. Which are the third of the organic groups? What do they embrace? Points of resemblance?]