s found in the curdled milk of the
whole Balkan peninsula, and even in the Don region of Russia."[41]

It is a short step from considerations like these to the adoption of the
_Bacillus bulgaricus_ as the most potent of the various lactic organisms
which have been examined, and which is likely to play such an important
role in the destiny of the human race. The _Bacillus bulgaricus_ may
claim to be the Bacillus of Long Life.

CHAPTER III

THE CHEMISTRY OF MILK

_The Composition of Milk._--Like all other organic substances, or those
built up in connection with the life processes of plants and animals,
milk is of complex composition. It is also very liable to change--every
one is acquainted with its tendency to "go bad." This instability is
more or less inherent in all highly organised chemical compounds, and,
indeed, it seems to be necessary that the materials used in growth and
nutrition should be very plastic in a chemical sense, in order, _e.g._,
that the constituents, say of a plant, may easily be transformed into
the substances of the body of the animal which feeds on it.

The perishable nature of milk--the food of young and growing animals--is
therefore essential, so that it may be changed easily into the blood,
bone, muscle, etc., so abundantly required in the early stages of
existence.

Milk is a complete food, and, therefore, naturally it is not a simple
chemical compound, but a mechanical mixture of a number of substances.
The present state of chemical knowledge on the subject does not permit
of its composition being given in detail, but for practical purposes,
such as those of measuring its purity and food value, this is not
necessary.

A proximate analysis, in which, at least, some of the ingredients are
lumped together, is sufficient, and has been adopted everywhere by
analysts. On this basis the average composition of cows' milk may be
stated as follows:

Per cent.
Water 87.50
Fat 3.50
Casein and albumen 3.65
Milk sugar 4.60
Ash 0.75
----
100.00
------

The constituents other than water added together form the "total
solids," and they amount to 12.5 per cent.

Per cent.
Water 87.5
Total solids 12.5
----