~Definition of Fat.~--The fats are all glycerides; that is, they are substances made up of combinations of fatty acids and glycerine, which constitute a definite group of chemical compounds, certain members of which are liquid in form, while others are solid, or semi-solid. The liquid fats are known as fatty oils. The fatty acids in which we are chiefly concerned in this study are: Butyric, Stearic, Oleic, and Palmitic. Most of the common fats owe their form and flavor to the type and amount of the various fatty acids of which they are composed. For example, butter is made up of ten fatty acids; but its soft, solid form is due to the olein and palmitin (glycerides of oleic and palmitic acids) which it contains; and its characteristic flavor, as well as its name, to its butyric acid content (about 5 to 6%). It is evident that the degree of softness or hardness of a fat may be determined chiefly by the amount of oleic acid in its composition. Most of the common oils with which we are familiar in food are composed chiefly of olein. Stearin (the glyceride of stearic acid) is the hardest of the fatty acids, while palmitin, although classed with the solid fats, is not so hard as stearin. Lard and butter are higher in olein and palmitin and are consequently semi-solid, while suet and tallow, consisting chiefly of stearin, are much harder than the other food fats. ~Characteristics of Fats.~--The fats are all insoluble in water, and only partially so in cold alcohol, but they dissolve readily in ether. As a rule, the fat occurring in the animal body is more or less characteristic of the species. For example, animals that live on land have a harder fat than those living in the water; warm blooded animals, harder fats than cold blooded ones (fish); and carnivorous animals, harder fats than herbivorous species. Fats are lighter than water, hence will float in it. An emulsion is a suspension of fat in a fluid, and the fat in this case must be very finely divided and mixed with some other material which will prevent a coalescence of the fat globules. In milk, which is one of the best natural emulsions, the additional substance is protein.