n, but preparations are made by
using solutions of anilin dyes as staining agents. These are of great
service in bringing out almost imperceptible differences. The art of
staining has been carried to the highest degree of perfection in
bacteriology, especially in the detection of germs that are found in
diseased tissues in the animal or human body.

In studying the peculiarities of any special organism, not only is it
necessary that these cultural and microscopical characters should be
closely observed, but special experiments must be carried out along
different lines, in order to determine any special properties that the
germ may possess. Thus, the ability of any form to act as a fermentative
organism can be tested by fermentation experiments; the property of
causing disease, studied by the inoculation of pure cultures into
animals. A great many different methods have been devised for the
purpose of studying special characteristics of different bacteria, but a
full description of these would necessarily be so lengthy that in a work
of this character they must be omitted. For details of this nature
consult standard reference books on bacteriological technique.

CHAPTER III.

CONTAMINATION OF MILK.

No more important lesson is to be learned than that which relates to the
ways in which milk is contaminated with germ life of various kinds; for
if these sources of infection are thoroughly recognized they can in
large measure be prevented, and so the troubles which they engender
overcome. Various organisms find in milk a congenial field for
development. Yeasts and some fungi are capable of growth, but more
particularly the bacteria.

~Milk a suitable bacterial food.~ The readiness with which milk undergoes
fermentative changes indicates that it is well adapted to nourish
bacterial life. Not only does it contain all the necessary nutritive
substances but they are diluted in proper proportions so as to render
them available for bacterial as well as mammalian life.

Of the nitrogenous compounds, the albumen is in readily assimilable
form. The casein, being insoluble, is not directly available, until it
is acted upon by proteid-dissolving enzyms like trypsin which may be
secreted by bacteria. The fat is relatively resistant to change,
although a few forms are capable of decomposing it. Milk sugar, however,
is an admirable food for many species, acids and sometimes gases being
generally produced.

~Condition when