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Page:Popular Science Monthly Volume 32.djvu/549

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Specific gravity 1·03
Total solids 12·50
Fat 3·20
Organic solids not fat 8·70
Mineral salts ·60
Water 87·50

The sample of milk selected for examination should be quite fresh, but never warm. The temperature should be about 60° F.

Of the several determinations required, that of specific gravity is at once the easiest and the most important. It is made either by means of the specific-gravity bottle or the hydrometer.

The specific-gravity bottle is a small glass flask which will contain a given weight of distilled water. The form used by the writer is shown in Fig. 1. It is adjusted to fifty grammes of water, and is accompanied PSM V32 D545 Fifty gramme specific gravity bottle.jpgFig. 1.—Fifty-Gramme Specific-
Gravity Bottle.
by a small brass weight to exactly counterpoise the weight of the flask. To determine the specific gravity of a sample of milk the bottle is completely filled with the milk and the stopper brought into contact with the fluid, care being taken that no air adheres to its under side. The stopper is then permitted to fall quickly into its bearing in the neck of the bottle. The excess of milk escapes through the fine perforation in the stopper. In this way an invariable volume of fluid is always weighed. The flask is then carefully wiped off, and the whole weighed, remembering, of course, to add the counterpoise weight to the scale-pan, in order to allow for the weight of the flask. The weight of the milk is obtained in grammes, and this multiplied by ·02 will give its specific gravity directly.

By the use of the hydrometer (or lactometer, as it is usually called, when graduated for milk), the determination of the specific gravity may be made much more rapidly, A convenient form of this instrument is shown in Fig. 2. It will require no explanation beyond brief mention of the system of marking employed. The upper graduation on the stem of the tube is marked 1, and is the point to which the instrument will sink when placed in pure, distilled water, at 60° F. The lower graduation is marked 1·05, and is the point to which the instrument will sink when placed in a fluid of that specific gravity. As normal milk averages 1·03, the variations on either side will be fully covered by these limits. A low specific gravity indicates that the