Page:Popular Science Monthly Volume 32.djvu/550

532 pump has been called into requisition, while a high specific gravity indicates a deficiency in cream. A popular form of lactometer, showing directly the proportion of water added to the sample, may be purchased of dealers in chemical apparatus for seventy-five cents, but some care must be exercised in its selection, since one instrument now offered the public has the point marked "skim milk" above that marked "pure milk." Evidently it should be below, as skimmed milk has a greater specific gravity than whole milk.

In some States the test of specific gravity by the lactometer is the only determination made. In spite of the importance of the test, however, it is not always conclusive. A very rich milk will show a suspiciously low specific gravity, while a skimmed milk, diluted with water to the proper density and colored to an agreeable cream-tint by "Richardson's Perfected Butter Color" or other dye, will escape so much as the breath of scandal.

The amount of cream is generally determined by permitting the sample to stand in a graduated jar until the cream separates, and then reading off the volume percentage directly. Centrifugal machines are also used, but a simpler test than either of these is usually sufficient. If a closed tube, blackened on the inside, is dipped into the sample, and slowly withdrawn, one can judge of the richness of the milk by the opacity of the film remaining on the tube.

To determine the total solids, five cubic centimetres of the sample of milk are placed in a small platinum dish of known weight, and the joint weight of milk and dish then obtained. The difference between the two will be the amount of milk taken for analysis. It is then evaporated to dryness on a water-bath without stirring. This will take about an hour, at the end of which time the dish will be found to contain a yellowish-white mass which shows a thin, transparent film on top and a honeycombed structure beneath. A number of cracks will extend throughout the mass, on account of the contraction on drying, but do not indicate that anything is amiss. The dish and contents are then put into an oven at 212° F., and at the end of half an hour are taken out and weighed. The known weight of the dish subtracted from the weight thus obtained gives the weight of the total solids. The ratio of this weight to the weight of milk taken for analysis will give the percentage value.

If it be desired to find the amount of fat, petroleum benzine is poured over the residue and is renewed at the end of an hour. After standing half an hour longer, the solvent is again decanted. The residue is thoroughly washed with benzine, and is dried in an oven at