6o MICRO-ORGANISMS AND DISEASE [CHAP. 



than the one at which the nutrient gelatine remains solid, 

 while others refuse altogether to grow in gelatine, or grow 

 only too slow. In the latter case no success can be looked 

 for, if those bacteria which form their colonies much faster 

 and are present in large numbers crowd out the others that 

 require a long time to come up. 



In such cases, particularly when one has to deal with 

 bacteria that do not grow in gelatine at the temperature at 

 which this latter remains solid, the same method of plate- 

 cultivation can be used, but substituting the gelatine by the 

 Agar-Agar peptone mixture above mentioned, previously 

 liquefied by heating, care must be taken not to proceed with 

 the inoculation of the Agar-Agar mixture before the tem- 

 perature has fallen to about 42° to 50° C. All other mani- 

 pulations remain the same. 



It is perhaps not unnecessary to state that if the Agar 

 mixture is of recent date — contains, therefore, while in the 

 tube condensation fluid, after pouring it out in a plate dish, 

 cooling this, letting the Agar set, and placing the cultivation 

 in the incubator at 37° C, in all probability there will again 

 appear condensation water in the plate ; as the colonies 

 begin to develop on the surface they will be swamped by 

 that water and the whole surface will become covered with 

 an indiscriminate film of growth. It is therefore advisable 

 to keep the plate- dish in the incubator inverted and in 

 slanting position. If, however, the Agar mixture used for 

 the plate-culture is of some standing this is not necessary. 

 But I keep also gelatine plates in an inverted condition in 

 the incubator for the first day, in order to avoid too great 

 a loss of water by evaporation ; care must of course be taken 

 that as soon as liquefying colonies appear and begin to 

 spread the plate-cultivation must again be placed upright. 



A method which I have found very useful for making 



