MODIFICATIONS OF THE PLATE PROCESS 59 



loopful is conveyed into a test-tube of liquefied gelatine, 

 which is then poured out into a Petri's capsule. 



Agar can be used instead of gelatine for the plate pro- 

 cess, but this medium requires greater care in preparing 

 the cultivations than is the case with gelatine. Agar be- 

 comes fluid at 90 C., and passes into the solid state at 40 C., 

 hence the liquefied mass must be cooled down to 40 before 

 it is inoculated, since at a higher temperature the micro- 

 organisms might be destroyed. The mass when inocu- 

 lated is poured out upon sterilised plates with the precau- 

 tions mentioned above, and as the water of condensation 

 which separates out renders the film of agar liable to slip 

 from the plate, it is prevented from doing so by dropping 

 some sealing-wax on the edge of the medium. But a more 

 convenient plan is to use Petri's capsules or Soyka's plates 

 to contain the mass. Agar plates have the special 

 advantage that they can be kept for a considerable time 

 at incubation temperature, and that they do not undergo 

 liquefaction. 



The roll process can also be applied to cultures on agar. 



To facilitate the isolation of micro-organisms, Dahmen 

 has devised an apparatus consisting of a double capsule, of 

 which the upper part extends beyond the lower. The latter 

 is placed on a glass plate and surrounded with an india- 

 rubber ring, on which the upper capsule rests securely. 

 The lower capsule is prepared with the inoculated nutrient 

 agar, placed in the centre of the rubber ring, and covered 

 over with the larger capsule ; the whole is then surrounded 

 with an india-rubber band and deposited in the incubator. 



The individual colonies form on the agar plate in the 

 same way as on the gelatine (except that they do not liquefy 

 it), appearing coloured or glossy, and showing characteristic 

 outlines. 



Plate cultures on serum and plover's egg albumen. Blood 



