SEPARATION BY GELATIN MEDIA 57 



culture is or is not pure. The suspected culture is plated (three 

 plates being prepared, as will be described). If all the colonies 

 are the same, then the culture may be held to be pure. 



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. 17). They are 

 usually 3 inches in diameter and 

 half an inch deep. The advant- 

 age of these is that they do not FIG. 17. -^tri's capsule, 

 require to be kept level by a (Cover shown partially raised.) 

 .special apparatus while the medium 



is solidifying, and can be readily handled afterwards without 

 admitting impurities. Whether plates or capsules 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. 18). 



1. Glass Capsules. While in certain circumstances, as when 

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

 of glass, Petri's capsules are to be preferred in- the usual labora- 

 tory routine for the above reasons. 



The contents of three gelatin tubes, marked a, b, c, 1 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 

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

 of the mixture may be transferred to the medium. Shake a 

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

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

 five loops to c. The plugs of the tubes are in each case replaced 

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

 three tubes are then poured out into three capsules. 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 capsules are labelled and 

 set aside till growth takes place. 



1 For marking glass vessels it is convenient to use the red, blue, or yellow 

 oil pencils made for the purpose by Faber. 



