Milk contains traces of many other substances, the most important of which are several enzymes which assist in its digestion.

General Properties of Milk.—The appearance of milk is known to every one; it ought to be a pure white opaque liquid, but very generally it is tinted a cream colour with anatto to give it an added appearance of richness. The average specific gravity is about 1.031; or, to put it another way, while a gallon of pure water weighs exactly 10 lbs., a gallon of milk weighs 10 lbs. 5 oz. It freezes at 31° F. and boils at about one third of a degree higher than water.

When milk is examined under the microscope, the fat is found to be distributed through it in a multitude of minute globules varying in size from 1/16,000th to 1/25,000th part of an inch, and occasionally they are much smaller and also much larger.

Fig. 1 is a micro-photograph showing the fat globules in whole milk. Fig. 2 is a micro-photograph of separated milk, and Fig. 3 a micro-photograph of cream, all under high magnification (450 diams.); from these figures the comparative number of fat globules present may be seen.

Fig. 1.—Micro-photograph of a Drop of Whole Milk, showing distribution of fat globules. (Magnified 450 diams.) Fig. 2.—Micro-photograph of Separated Milk, showing the almost complete absence of fat globules as compared with whole milk. (Magnified 450 diams.)

Fig. 3.—Micro-photograph of Cream, showing agglomeration of fat globules. (Magnified 450 diams.)

Fats distributed through a watery liquid in this finely divided condition form together what is called an emulsion, in which the particles of fat are kept apart by surface tension. The specific gravity of milk fat averages 0.93, and compared with water weighing 10 lbs., a gallon of fat would weigh 9 lbs. 5 oz. It is thus considerably lighter than the other constituents, and when milk is left at rest, the fat globules gradually rise to the top and float there, forming cream. The difference in specific gravity between cream and milk is taken advantage of in the mechanical separator, now so much used, and which makes such a thorough separation between the two. Cream is an article of the most varied composition, according to the ideas of the person who produces it, but it ought to contain at least 20 per cent. of butter fat, and may be made with a much larger percentage if necessary. When cream is agitated in a particular way, as by churning, the surface tension of the particles is overcome, and they run together into a mass which forms butter.

The casein of milk is not held in solution in the ordinary sense, but in a peculiar state of suspension called the colloidal condition, practically the whole of it remaining behind when milk is filtered through clay filters.