Report on the Bacteriology of Water. 367 



clearly that the individual colonies are weakened by the light-action, 

 and the results with the original tubes (given below) may also be 

 consulted. 



Turning now to the original broth cultures, placed at 20 22 C. 

 In eighteen hours the darkened tube was distinctly turbid, whereas 

 that exposed to the sun was perfectly clear ; and the same was the 

 case at the end of forty hours. In sixty-four hours both tubes 

 were turbid, but the exposed one far less so than the other. Later 

 on it was impossible to distinguish between them. 



It seems to me impossible to avoid the conclusion that the light- 

 action so weakens the metabolism of the insolated cells that they 

 grow and divide more slowly, and dissolve the gelatine more feebly, 

 and possibly this weakening effect is transmitted to the cells to 

 which they give rise by division. As time passes, however, the cells 

 gradually recover their vigour in the dark, and where plenty of food 

 material is accessible. 



That this latter statement is true the following experiment 

 proves : On October 18 I took the two broth-tubes of Colony /3, 

 referred to above, both of which had been in the dark, side by 

 side, since October 6. Broth-tubes were exactly similar at the 

 beginning, as we have seen, but one of them, had been exposed, on 

 October 6, for seven hours to the sunlight. 



On making stab-cultures from these tubes, no difference could be 

 detected between their behaviour, both began to liquefy the gelatine 

 in forty-eight hours in the typical, thistle-head funnel form. 



This seems to show clearly, also, that the broth has not been in- 

 jured as a medium for culture by its exposure to light. 



In order to meet the objection that the above results were due 

 to the using of broth-cultures, I repeated them as follows on 

 October 10 : 



Two tubes of sterile water, infected from the turbid broth-culture 

 kept in the dark since October 6, were suspended, as already de- 

 scribed one exposed over a mirror to the bright direct sunlight from 

 10 A.M. to 2 P.M., the other wrapped up, and placed beside it. At 

 2 P.M. plates and stab-cultures were made from each tube. 



The results were the same as before. The plates from the unex- 

 posed tubes showed numerous liquefying colonies in forty-eight hours, 

 and were completely liquefied on the third day ; the plates from the 

 insolated tube showed only very few colonies, and these not till the 

 third day, and they were denser, more granular, and without any 

 signs of liquefaction for several days. To take a concrete case : a 

 plate made with 1 drop (1/30 c.c.) from the dark tube showed about 

 5000 colonies in forty-eight hours, and these were already beginning 

 to run ; the whole of the gelatine was liquefied in another twenty- 

 four hours; the corresponding plate made with 1 drop (1/24 c.c.) 



