62 METHODS OF CULTIVATION OF BACTERIA. 



also be used to test whether a tube 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 cultures 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 advantage of these is that 

 they do not require to be kept 



* b l a spedal apparatus 



while the medium is solidify- 

 ing, 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). 



i. Glass Capsules. While in certain circumstances, as 

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

 to use plates, in the usual laboratory routine Petri's cap- 

 sules are to be preferred for the above reasons. 



The contents of three gelatine 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. 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 mixture 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 



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

 yellow pencils made for the purpose by Faber. 



