54 METHODS OF CULTIVATION OF BACTERIA. 



Either simple plates of glass 4 inches by 3 inches are used, 

 or, what are more convenient, circular glass cells with similar 

 overlapping covers. The latter are known as Petri's dishes or 

 capsules (Fig. 19). They are usually 3 inches in diameter and 

 half an inch deep. The advantage of these is that they do not 

 require to be kept level by a special apparatus while the medium 

 is solidifying, and can be readily handled afterwards without 

 admitting impurities. Whether plates or Petri dishes are used, 

 they are washed, dried with a clean cloth, and sterilised for one 

 hour in dry air at 170° C, the plates being packed in sheet-iron 

 boxes made for the purpose (see Fig. 20). 



I. Petri's Dishes. — While in certain circumstances, as when 

 the number of colonies has to be counted, it is best to use plates 

 of glass, in the usual laboratory routine Petri's dishes are to be 

 preferred for the above reasons. 



The contents of three gelatin tubes, marked a, b, c} are 

 liquefied by placing in a beaker of water at any temperature 

 between 25" C. and 38° C. Inoculate a with the bacterial 

 mixture. The amount of the latter to be taken varies, and can 

 only be regulated by experience. If the microscope shows 

 enormous numbers of different kinds of bacteria present, just 

 as much as adheres to the point of a straight platinum needle 

 is sufficient. If the number of bacilli is small, one to three 

 loops of the mfxture may be transferred to the medium. Shake 

 a well, but not so as to cause many fine air-bubbles to form. 

 Transfer two loops of gelatin from a to b. Shake b and trans- 

 fer two loops to c. The plugs of the tubes are in each case re- 

 placed and the tubes returned to the beaker. The contents of 

 the three tubes are then poured out into three dishes. In doing 

 so the plug of each tube is removed and the mouth of the tube 

 passed two or three times through the Bunsen flame, the tube 

 being meantime rotated round a longitudinal axis. Any organ- 

 isms on its rim are thus killed. The dishes are labelled and 

 set aside till growth takes place. 



The colonies appear as minute rounded points, whitish or 

 variously coloured. Their characters can be more minutely 

 studied by means of a hand-lens or by inverting the dish on 

 the stage of a microscope and examining with a low power 



1 For marking glass vessels it is convenient to use the red, blue, or yellov^ v^ax 

 pencils made for the purpose by Faber. 



