g 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 a fair idea of its quality.
The taste and smell afford some guide, as also the general appearance.
To judge of the latter, place some of the milk in a tumbler or other
clear glass vessel. If the milk is of good quality it will be quite
homogeneous and opaque. Any flocculent matter indicates either disease
in the cow or that the milk is old and bacteria have multiplied in it
and altered its composition. When the milk has stood long enough for the
cream to rise freely, the latter should form a perfectly homogeneous and
strongly defined layer on the top. The quantity of cream may be measured
in a creamometer, which consists of a small glass cylinder graduated at
the top (Fig. 5). It is filled with milk to the top graduation line, and
when the cream has risen, the percentage quantity of the latter which
has separated can be taken off.

[Illustration: FIG. 5. The Creamometer]

The colour should be like that of porcelain, but, as alr