52 METHODS OF CULTIVATION OF BACTERIA 



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 level by a 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 ; in the usual laboratory routine Petri's capsules are to 

 be preferred for the above reasons. 



The contents of three gelatin tubes, marked a, 6, 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 

 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 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. 



For accurate work it will be found convenient to carry out 

 the dilutions in definite proportions. The following is the pro- 

 cedure which we have found very serviceable. In a number of 

 small sterile test-tubes '95 c.c. sterile water is put. To the first 

 tube we add '05 c.c. of the bacterial mixture. The contents of 

 the tube are well shaken up, and the pipette is sterilised by 

 being washed out with boiling water. It is allowed to cool, and 

 05 c.c. of fluid is transferred from the first tube to the second. 

 By a similar procedure '05 c.c. is transferred from the second to 



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

 oil pencils made for the purpose by Faber. 



