071 Milk and Milk-Products. 
379 
each supposed to contain one organism in the sterilized milk, 
only five curdled, whilst five remained fluid, sweet, and in 
every way unchanged for a number of weeks. The curdling 
was in every case accompanied by the development of a definite 
kind of organism, while in the five samples which remained 
sweet this was entirely absent. 
A more certain way of proceeding, when a separation of 
different kinds of micro-organisms is contemplated, is that 
suggested and practised by Koch : — 
Sterilized nutritive gelatine is spread out upon a glass plate. A platinum 
pin previously ignited is dipped into tlie milk under examination, and with 
the pin a line is drawn upon the gelatine plate ; this is then exposed to a 
temperature suitable for the development of micro-organisms. All these 
operations are carried out under such precautions as will preclude the possi- 
bility of outside influences, which, if not entirely prevented, are certainly 
diminished to a very great extent. 
After the lapse of a few days, finely-drawn white lines or 
detached spots are noticed, each spot representing a colony of 
micro-organisms developed from a single germ. The lines 
are formed of colonies closely situated to each other. As these 
colonies develop, differences in the manner of their growth will 
be observed. Many will be found to resemble one another, 
whilst in others the differences are more pronounced, and upon 
closer observation micro-organisms different from the rest will 
be found. 
Should the material for inoculation have been taken from 
sour milk, by far the greater number of the colonies will be 
of a very similar character. On the surface of the gelatine they 
will grow to the size of a lentil, of a white porcelain-like colour 
and encircled by smooth, not dented, outlines. Other colonies, 
which at the earlier period of growth may exhibit a similar 
character, show decided differences in their further development. 
In order to prove whether the germs which form the colonies 
described have any connection with the lactic fermentation of 
milk, it is necessary to prepare perfectly sterilized milk. This 
can be done by keeping small samples contained in suitable 
vessels in boiling water, or better still, in steam of the tem- 
perature of boiling water for several hours, or by heating the 
samples up to 160° Fahr. for one hour daily on five consecutive 
days. Samples thus treated and secured against contamination 
from outside will undergo no perceptible change for as long 
a period as twelve months. If, however, the point of a pin be 
brought into contact with one of the colonies described, and be 
then dipped into the milk, and the latter exposed to a tempera- 
ture ot 100° Fahr. (always preventing contamination from 
other sources), the milk will be found to become coagulated 
