the preparation of casein by this method, and the washed and dried
precipitate is used very extensively in the arts for such varied
purposes as the manufacture of billiard balls, paints, cements, etc.

The clear liquid which separates when milk is curdled with rennet is
called whey, and contains the milk sugar and mineral salts. The sugar is
manufactured from it on a limited scale, and is used as an ingredient in
infant foods, and as a convenient medium in certain medical
preparations. In Sweden a kind of cheese is made from whey, but the
great bulk of it everywhere is used for feeding pigs.

The comparative composition of different varieties of milk is given in
the following table:

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Human Cow Buffalo Goat Sheep Mare Ass Reindeer Whale
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Water 88.32 87.75 82.57 86.34 81.08 90.38 90.30 67.7 60.47
Fat 3.43 3.40 7.63 4.25 7.67 1.00 1.30 17.1 20.00
Protein 1.55 3.50 4.69 4.40 6.08 1.98 1.80 10.9 12.42
Milk Sugar 6.44 4.60 4.30 4.26 4.26 6.28 6.20 2.8 5.63
Salts 0.26 0.75 0.81 0.75 0.91 0.36 0.40 1.5 1.48
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Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
-------------------------------------------------------------------------
Specific 1.032 1.0315 1.033 1.033 1.038 1.034 1.033 ... ...
Gravity
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[Illustration: FIG. 4 is a photograph of two Petri dishes,
which have been inoculated with ordinary milk (A), and
milk that has been subjected to sterilisation (B). The
whitish bacterial colonies on A are due to enormous
numbers of organisms, while B is quite free from such
growth.

For the production of a reliable lactic food, it is
essential that certain precautions as to the treatment of
the milk, and the maintenance of a suitable temperature
during the growth of the lactic bacteria, should be
observed.

In the first place, milk immediately after extraction from
the cow contains only a few organisms, but these multiply
so rapidly that in a few hours the bacterial content may
amount to many millions per ounce. In preparin