It consists therefore of 87% water and 13% “solids,” 4 of which are fat and 9, “solids—not fat.” =Butter-Fat= is found in milk in the form of minute globules suspended in the liquid. As fat or oil is lighter than water (has less specific gravity) it naturally floats and the fat globules therefore soon rise to the top when milk stands at rest, forming a layer of _cream_ which may be skimmed off from the _milk_. In chemical composition butter-fat is very nearly the same as other animal and vegetable fats, but the slight variation may make a lot of difference in digestibility and palatability as well as in nutritive value, and it is a mistake to consider Oleomargarine, Butterine, Nut-Butter and other substitutes for butter “just as good” because a chemical analysis shows them to contain “almost” the same elements or compounds. These products may be good and legitimate if sold for what they are, but should never be allowed to be sold or served for what they are not, genuine butter. [Illustration: Composition of cow’s milk ] =Casein and Albumin= are the proteids or nitrogenous matters of the milk, in chemical composition and in food value much the same as the protein in beans and peas or in meat, or the white of eggs. Casein is present in much larger quantity than albumin and is distinguished from the latter by being precipitated by rennet, which has no effect on albumin. Casein may also be precipitated by acids, while it required boiling as well as acidity to throw the albumin out of solution. =Milk-Sugar= is related to the vegetable sugars, glucose, cane sugar, etc., and remains in solution in the whey which separates out from the cheese-curd when milk is curdled with rennet or acids. The whey also contains the mineral matters or _ash_, which consists largely of phosphate of lime, of use to the young in building up bones. When whey is boiled down to a thick syrup and left to stand, the sugar will crystallize out and may be separated from the liquid, still holding in solution the mineral matters. [Illustration: Fat globules as they appear under the microscope ] =Mineral Matters.=—These may be gathered as _ash_ by boiling off the water from this liquid or from the whole milk and burning the rest, as all the constituents except the ash belong to what in chemistry is called organic matter, which disappears in the air by burning. [Illustration: Dr. S. M. Babcock of Madison, Wis. ] HOW TO TEST MILK Milk may be adulterated or decreased in value either by skimming or by watering. In either case the percentage of the most valuable constituent, the butter-fat, is diminished. It is upon this fact that the most practical test, the Babcock, is based. [Illustration: Babcock test outfit ]