100 TEXT-BOOK OF BACTEEIOLOGY. 



As we know, nutritive agar does not liquefy under about 90° C; 

 and it returns to its solid state at about 38° C. As we cannot put 

 germs into a solution of 40° or above without killing them, we are 

 obliged, after letting the agar melt completely in boiling water, to 

 allow it to cool down again gradually to 40° C. exactly. Then the 

 tubes are inoculated and the dilutions made in the manner de- 

 scribed. Yet this must be done as quickly as possible, for if the 

 temperature sinks but a trifle lower the agar becomes solid and 

 the labor is all in vain, for we cannot, for the above-explained rea- 

 son, melt the inoculated food agar over again as we could the 

 gelatin, without running the risk of destroying the material in- 

 oculated. 



The best way is to pour the contents of the test-tube speedily 

 into sterilized - Petri dishes; in a few moments the agar is hard, 

 and if care has been taken to tilt the dishes properly', it covers the 

 bottom in an even, uninterrupted layer. All other processes, in- 

 cluding the original one of Koch and the rolling process of Esmarch, 

 are either difficult or impossible with agar instead of gelatin. 



Troublesome as is the manipulation of agar compared with that 

 of gelatin, yet agar is indispensable for many purposes, and espe- 

 cially in the case of organisms which only thrive or which thrive 

 best at the temperature of the incubator. Place the dishes with 

 their contents in the incubator, and leave them there for from 

 twenty-four to forty-eight hours. 



The third solid and transparent food medium cannot, as has 

 already been said, be employed in like manner, since it cannot be 

 brought into a solid state by low temperature, but, on the contrary, 

 requires a high temperature to harden it; so high a temperature, 

 indeed, that most bacterial germs would be destroyed by it. 



Serum can, at best, only be employed for stick cultures; we 

 pour it into little dishes, in which we make it consolidate, and then 

 inoculate its surface. Generally we are obliged to fairly rub the 

 inoculating matter into the jelly-like mass, and there is but a very 

 bare possibility of afterward distinguishing the separate develop- 

 ing germs. 



VI. PLATE CULTURES— PETRI DISH CULTURES— ESMARCH ROLL- 

 TUBE CULTURES. 



Sooner or later, according to the temperature of the place m 

 which the plates are kept, the colonies develop in the gelatin. Gen- 

 erally three or four days elapse before they have acquired a toler- 

 able size and are visible to the naked ej^e. At the same time it 



