PREPARATION OF CULTURE MEDIA II 



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 our dilution has been properly made, we shall have 

 separated each organism from its neighbours, and each separ- 

 ate 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 colour- 

 less, one may be round and the other angular, etc. 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. 64, and a simplified method, which is much more 

 convenient, is described on p. 17. 



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 pro- 

 cess, adding a definite measured quantity of the fluid to the 

 culture-tube of liquefied gelatin. The number of colonies 

 which develop is counted, and this gives us the number of 

 bacteria in the sample of fluid. For example, if -^o c.c. diffused 

 throughout a tube of melted gelatin and poured out into a 

 thin film produced twenty colonies, it follows that I c.c. of the 

 fluid contained 200 bacteria. This is a brief description of the 

 essentials of the method adopted in the quantitative examina- 

 tion of water and other fluids. 



Requisites for the Manufacture of Gelatin. i. 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 I2j grammes of 

 gelatin for each 100 c.c.; allow to soak for an hour or more, 

 and then heat until the gelatin is dissolved. Continue the heat, 

 and render the medium faintly alkaline, just as was done in the 

 preparation of broth. Now filter through a moistened filter- 

 paper. To avoid the setting of the gelatin during the filtra- 

 tion it is best to use a double-jacketed funnel containing hot 



