PREPARATION OF CULTURE MEDIA. 17 
our dilution has been properly made we shall have 
separated each organism from its neighbours, and each 
separate germ will grow up into a ‘“‘colony ” which will 
soon be visible to the naked eye. In all probability we 
shall be able to see that these colonies are of two kinds; 
one may liquefy and the other not, one may be coloured 
and the other colourless, one may be round and the 
other angular, &c. Samples of each sort of colony 
are then transplanted to fresh culture tubes and again 
incubated. An example of this process is given on p. 70. 
A slight modification of this process enables us to 
make an estimate of the number of living bacteria 
which is present in a given fluid. To do this we have 
to follow out the above process, adding a definite 
measured quantity of the fluid to the culture tube of 
liquefied gelatin. The number of colonies which de- 
velop is counted, and this gives us the number of 
bacteria in the sample of fluid. For example, if one- 
tenth of a cubic centimetre diffused throughout a tube 
of melted gelatin and poured out into a thin film 
produced twenty colonies, it follows that one cubic 
centimetre of the fluid contained two hundred bacteria. 
This is a brief description of the essentials of the 
method adopted in the quantitative examination of 
water. 
Requisites for the manufacture of gelatin :— 
1. Broth. 
2. Gelatin. (Coignet’s gold label gelatin is best, but 
any good brand will do). 
3. Dilute solution of sodium carbonate. 
4. Litmus papers. 
5. Flasks, stirring-rod, funnel, and plugged test-tubes 
as for broth. 
Method.—Measure the broth and add to it 12% 
c 
