92 Fermentation and Kindred Phenomena, 



Consider the effect of this operation. The organisms present 

 in the liquid with which the gelatine was inoculated are presum- 

 ably equally distributed, and if only a few are present (which 

 can be ensured by diluting the liquid used for inoculating) each 

 individual is separated from another by a considerable space. 



In course of time (only a few hours under favourable conditions) 

 each organism reproduces itself, eventually producing a colony, 

 and this colony liquefies the gelatine at a particular spot or 

 causes an opacity. Hence we may be certain that liquid 

 taken from this spot contains only one species of organism, and 

 with this we can inoculate our sterile liquid and so obtain a 

 pure culture. This beautiful method of Koch's has been em- 

 ployed by him for isolating and investigating many of the 

 organisms of disease, and can be used for measuring the number 

 of organisms in air, water, and other fluids. 



Now, having inoculated our nourishing medium with the 

 organism we wish to study, we have next to place it under 

 favourable conditions for their growth, and as a rule that means 

 a steady temperature (on an average about as high as that of 

 our bodies). The apparatus used is called an " incubator," and 

 is simply a box with double walls, the interspace being filled 

 with water which is kept at a constant temperature by a gas 

 flame, automatically controlled by a " thermostat," so that the 

 temperature inside the box (where the culture is kept) never 

 varies by more than a few degrees. 



Such in a few words are the chief methods employed in 

 "bacteriological" research, and with their introduction more 



