42 MICRO-ORGANISMS AND DISEASE. [CHAP. 



40 C., then the plug is lifted and the gelatine inoculated 

 with a mere trace of the bacterial mixture, either by means 

 of the point of the capillary pipette or of the overheated and 

 cooled point of a platinum wire ; the plug is replaced and the 

 gelatine shaken so as to distribute uniformly the bacteria 

 that had been introduced. A shallow glass dish with flat 

 bottom and ground edge, and covered with a similar but 

 slightly larger dish, has previously been sterilised in the oven 

 and then allowed to cool ; the liquefied nutrient gelatine 

 inoculated with the trace of the bacterial material is then 

 poured out into the lower dish so as to form a thin layer 

 at its bottom ; the lifting off of the dish-cover, the pouring 

 in of the gelatine, and the replacing of the cover, ought to 

 occupy only a moment. 



This apparatus is then placed on a glass plate, to which, 

 by means of greased edge, a bell-glass can be fixed, on the 

 interior of which is a piece of wet blotting-paper. In this 

 way a closed moist chamber is established. The whole is 

 then put into an incubator, the temperature of which does 

 not reach above 21 or 22 C. (or the temperature of the 

 room in the warm months), in order to insure the gelatine 

 setting and remaining so. If a trace of material containing 

 various species of bacteria is thus distributed into several cc. 

 of gelatine, each species will start a separate colony after a 

 few days' growth, and the individual colonies, if different, 

 will be apparent by different characters, according to shape, 

 colour, size of the colonies, and according to whether they 

 liquefy the gelatine or not during their growth. In order to 

 insure success, it is necessary to infect the original gelatine in 

 the test-tubes with only a trace of the bacterial mixture ; if 

 too many bacteria are introduced, their colonies sprouting up 

 are too numerous and soon become confluent. But if the 

 experiment is successful, from the individual and separate 



