Report ou the Bacteriology of Water. 415 



At 1.40 another filament in this drop was selected and measured as 

 follows, from 1.48 P.M. to 3.10 P.M. : 

 Doubling period : 



58-5117 M = 1-48 P.M. 2.42 P.M. = 54 minutes at 31 -531 75 C. 



Calculated back : 



170 -585 -25 M - 2.16 P.M. 3.10 P.M. = 66 minutes at 31 '532 C. 



A review of these broth-cultures bears out the general truth of the 

 previous statements, and justifies the following additional conclu- 

 sions : 



1. The growth in broth is more rapid than in the weak gelatine, 

 and the doubling periods, especially at lower temperatures, are corre- 

 spondingly shorter. 



2. A curious technical difficulty arises, and often gives great trouble 

 where broth is used (or when, at high temperatures, the gelatine 

 liquefies). On opening the side windows to rearrange the object for 

 measurement a slight draught is produced, and a cooling of the 

 cover-slip ensues ; this causes condensation of the vapour in the cell, 

 and, if a drop catches the edge of the hanging drop, currents are 

 produced, and the filament, if short and floating free, may move out 

 of the field. Slight and slow movements may be followed, and all go 

 on as before, but rapid ones are fatal to the purpose. 



3. Several of the cultures suggest that a period of exhaustion 

 gradually supervenes as the culture ages ; it remains to be seen how 

 far this is due to one or other of the following possible causes : (1) 

 mere using up of the food-materials; (2) fouling of the drop by 

 excreta ; (3) or by C0 accumulating as the oxygen is consumed in 

 respiration. 



C. Cultures in Broth + 1 per cent. Gelatine. 



On August 1 spores were sown as before, broth + 1 per cent, 

 gelatine, and in at 9.30 A.M., at temperatures as follows : 



2.28 = 21-1 



