VIABLE BACTERIA IN YOUNG CULTURES 413 



throughout the tube. In order to count the number which has 

 developed, a series of circles, 1 cm. apart, is liphtlj^ drawn around 

 the tube with an oil-marking pencil, and a longitudinal line 

 drawn from top to bottom intersecting each circle at right angles. 

 The actual counting is performed by means of a small magnifj'- 

 ing glass, the tube being examined against a dark back-ground 

 illuminated by a partially concealed electric bulb. The contents 

 of each circle are observed in turn from above downwards, till the 

 total number of colonies in the tube has been enumerated. This 

 procedure is greatly facilitated by the registration of each colony 

 on a small counting machine — a process which has the double 

 advantage of not only saving one the trouble of memorizing the 

 figures, but also of eUminating the personal factor in counting, 

 for one has no idea of the actual number till the tube has been 

 completely examined. 



In using roll tubes for this purpose one or two small points 

 should be noticed. The temperature of the agar at the time of 

 inoculation may be allowed to vary between 41° and 50°C.; no 

 deleterious effect of a temperature even as high as 55°C. could be 

 noticed in experiments made to determine this point. Further, 

 it is well to allow the agar tubes after melting to remain in a water 

 bath at 45°C. for five to ten minutes, so as to allow of the con- 

 densation of some of the excess moisture suspended in the air of 

 the tube, otherwise it may be deposited on the surface of the agar 

 after rolling, and lead to an undesirable amount of spreading. It 

 is to allow any such moisture to run down on to the cotton plug 

 and so be absorbed that the tubes are incubated in an inverted 

 position. Finally to prevent excessive condensation, it is ad- 

 visable to place the tubes in the incubator as soon after rolling as 

 possible. 



Comparison of tubes and plates 



The question will naturally arise as to the reason for the 

 abandonment of plates in favor of tubes. In reply the following 

 factors may be adduced: (1) The prohibitive cost of plates when 

 large numbers are required for use. In many experiments 40 

 and 50 tubes have been used at a time, and as these experiments 



