VIABLE BACTERIA IX YOUNG CULTURES 



425 



in which the emulsions contained as many as 30 to 170 per drop. 

 In all 2G experin;ents were performed. In each experiment the 

 number of colonies which developed in the 5-, 10-, 15-, and 20- 

 drop tubes was calculated as a percentage of the number which 

 develoi)e(l in the 1-drop tubes, after they had been rendered com- 

 parable with these by dividing by the number of drops they re- 

 spectively contained. An example will make this clear; the num- 

 ber of colonies given represents the arithmetic mean of the counts 

 of the number of tubes used: 



Here it is seen that the greater the number of organisms inocu- 

 lated per tube the lower is the actual percentage which develops. 

 In the 1-drop tubes, 77 colonies develop; theoretically therefore, 

 in the 20 drop tubes 1540 colonies should have appeared, but in 

 point of fact only 1173.8 were observed; that is, only 76.23 per 

 cent of the baciUi inoculated formed colonies. The results of 

 each experiment were plotted as a curve, the number of colonies 

 per tube being arranged along the abscissae, and the percentage 

 which developed along the ordinates. In the later curves, start- 

 ing, for instance, with 100 colonies in the 1-drop tubes, the base 

 could not be taken as 100 per cent, but had to be brought into 

 relation with the results of the earlier curves; reference to these 

 showed that in a tube containing 100 colonies only 97.6 per cent 

 of the bacilli inoculated had developed, and therefore this figTire 

 was taken as the starting point for the curve, the other figures in 

 the curve being taken as percentages of this number. Of course, 

 in the earlier curves, when only a few bacilli were being dealt with 

 per tube, the results were very irregular owing to the large sam- 

 pling error involved ; it was hoped that by taking the arithmetic 

 mean of the counts in 16 to 20 tubes, this would be ob\-iated, but 



