It is this state of suspension of the casein which makes milk opaque, but the opacity is considerably increased by the emulsified fat.

The coagulation of the casein in milk by the addition of rennet has already been referred to. Acids, either mineral or organic, also precipitate it in the form of flakes. Skimmed milk is now largely used for 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:
Human.
Cow.
Buffalo.
Goat.
Sheep.
Mare.
Ass.
Reindeer.
Whale.

Water
88.32
87.75
82.57
86.34
81.08
90.38
90.30
67.7
60.47
Fat3.43 3.40 7.63 4.25 7.67 1.00 1.30 17.1 20.00
Protein1.55 3.50 4.69 4.40 6.08 1.98 1.80 10.9 12.42
Milk Sugar6.44 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

Total
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.0
100.00

Specific Gravity
1.032
1.0315
1.033
1.033
1.038
1.034
1.033
...
...

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 preparing a pure culture of any specific organism, then, care must be taken to destroy all the bacteria that have accidentally found their way into the milk, inoculating with the organisms it is desired to cultivate. This is best accomplished by heating the milk to the boiling-point of water for about thirty minutes, by which time almost all the undesirable bacteria have been killed.

The milk of the cow differs a good deal from human milk, and where the former is used for the feeding of children it is usual to add milk sugar to it, and otherwise alter it to bring its composition more in harmony with the human article. The high concentration of the milk of the reindeer and the whale is noteworthy. Perhaps this may be due to the low temperature conditions in which these animals live, necessitating strong nutriment to enable their young to make proper progress in growth and development. On the other hand, the milk of the ass is poor in quality, and probably on this account it is more readily assimilated by those of weak digestion, to whom it is sometimes recommended. Goats' milk is richer than either cow or human milk, and its nourishing properties are well known. The goat is usually free from tuberculosis and other diseases which affect the cow, and its milk is therefore a very safe article to use.

The Analysis of Milk.—While the analysis of milk can only be made by a competent chemist, there are a number of simple tests and observations by which any intelligent person can obtain