16 BACTERIOLOGICAL DIAGNOSIS. 
the former amount is used in the winter, the latter in 
the summer. For general purposes 12} per cent. may 
be used in all cases. 
The special advantages of gelatin as a culture 
medium are two-fold. In the first place a great many 
organisms grow in or on it in a characteristic way, so 
that a bacteriologist may be able to identify the organ- 
ism by inspection of the culture. This arises partly 
from the fact that some bacteria produce a ferment 
which digests gelatin just as pepsin does; these bac- 
teria “liquefy ” the gelatin, and the distinction between 
the bacteria which have and those which have not this 
property is very important for purposes of diagnosis. 
Further, some bacteria liquefy rapidly and others 
slowly, and this is another important point in the 
identification of a germ. 
In the second place, the gelatin medium may be 
melted at a temperature (about 25° C.) at which 
bacteria are not killed. This fact is made use of in the 
isolation of bacteria from a fluid which contains several 
species by the process known as “lating.” Suppose, 
for instance, that we find by microscopic examination 
that a specimen of pus contains two different species of 
bacteria (perhaps a bacillus and a coccus) and we wish 
to obtain the two organisms in pure culture so that we 
can ascertain their nature and properties. We take a 
tube of gelatin and melt it by placing it in warm water, 
and then inoculate the medium with a minute quantity 
of the pus.* We then shake it so as to distribute the 
organisms throughout the melted fluid, and then pour 
the latter into a flat dish (Petri’s plate) so that the 
gelatin flows out into a thin film and then sets. If 
* Another and preferable plan is to inoculate the medium first and 
to melt it atterwards. 
