PREPARATION OF CULTURE MEDIA 13 



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 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, 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. 120, and a 

 simplified method, which is much more convenient, is described 

 on p. 121. 



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 develop is 

 counted, and this gives us the number of bacteria in the sample 

 of fluid. For example, if -^^ 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 bac- 

 teria. This is a brief description of the essentials of the method 

 adopted in the quantitative examination 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 12,^ grammes of 

 gelatin for each loo 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 filtration, it is best 

 to use a double-jacketed funnel containing hot water, but if this is 

 not at hand the whole apparatus (flask and funnel) may be placed 

 in the steam sterilizer (the lid being kept off to avoid the drops 



