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. Brothi 



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 



