Report on the Bacteriology of Water. 407 



From all this it must be inferred that temperatures from the 

 minimum to the optimum are antagonistic to the injurious light- 

 action, and are the more effective in this respect the nearer they are 

 to the optimum ; temperatures above the optimum, on the other hand, 

 help in the destructive light-action, or, rather, co-operate with the 

 light in killing, or in retarding the growth. 



(3.) The third external factor which helps to complicate these 

 curves is the nature of the food-medium, and here, again, it has been 

 extremely difficult to obtain quantitative results. The following 

 general conclusions seem dedncible from the experiments. 



Under otherwise similar conditions in the dark the doubling period 

 seems to be lengthened by the addition of gelatine, and the optimum 

 temperature raised somewhat. How far this is due to impeded access 

 of oxygen, owing to its slower diffusion through a more solid 

 medium, or a similar impeding of the action of solvent enzymes, or 

 to mere mechanical opposition to growth, or diminished access of 

 water or food solution to the organism, could not be decided. 

 Experiments with stiff gelatine at lower temperatures suggest the 

 co-operation of all these obstacles ; at higher temperatures it seems 

 probable that no mechanical opposition to growth occurs, and that 

 since the warm fluid gelatine absorbs much water, the difficulties 

 regarding that agent disappear also, but ifc is not improbable that 

 diffusion is still slower than in broth. 



Whether the shorter doubling period and lower optimum in broth 

 are due entirely to the absence of the above obstacles, or to the food- 

 materials being really in a more suitable condition for direct assimi- 

 lation by the cells, must also remain undecided. That the latter 

 factor is not, unimportant, however, is borne out by a few experiments 

 made with Jess suitable food-liquids e.g., glucose solutions and is, 

 of course, likely on other ground*. 



(4.) That access of oxygen is necessary for the growth follows from 

 direct experiments, and also from the extraordinary depression of the 

 curve when another oxygen-consuming bacterium gained access to 

 the drop. Experiments where KHO was put into the arms of the 

 cells did not support the idea that it is the mere accumulation of 

 C0 2 , due to respiration which ultimately causes the depression of the 

 curve at high temperatures, however, and further experiments are 

 needed to show how far, if at all, the organism can endure partial 

 pressures of oxygen. 



(5.) Some of the experiments, especially at higher temperatures, 

 indicate the importance of water, the general necessity of which will 

 of course not be disputed. 



The curves seem to be affected by sudden dilutions of the drops, 

 owing to condensation of water on the cover-slips of the culture-cells. 

 How far this is due to mere dilution of food : matcrials, enzymes, (fee.. 



