1894.] on the Action of Light on Bacteria and Fungi. 277 



Now the special interest of this new line of investigation I have 

 been pursuing turns largely on the fact that this particular bacillus, 

 which is not uncommon in the river Thames, is by no means a very 

 sensitive one to light, as compared with several others known to me. 



For if such a form can be so easily destroyed in a few hours by 

 the light from an April sky passing through several inches of water 

 and two sheets of glass, we may fairly conclude that more sensitive 

 forms are easily destroyed in the river, exposed to the rays of a 

 summer sun during the long days of June, July and August. 



As matter of fact — and the fact has been confirmed by many other 

 observations — I found that in spite of the high temperature of the 

 past summer, which we should expect to favour the development of 

 bacteria ; and in spite of the river being low, and presumably more 

 concentrated as a food-solution for bacteria ; and, further, in spite of 

 the fair expectation that, per unit of volume, the water ought to contain 

 more bacteria than during the cold months of October, November and 

 December, when the river is more diluted, and so on ; the number of 

 bacteria per cubic centimeter is decidedly and markedly less in the 

 summer than in the winter months ! 



This is very clearly shown by the statistical curves I now throw 

 on the screen. 



[Curves of average number of bacteria per 1 cubic centimeter of 

 Thames water in August, October and December, 1893.] 



Now I am perfectly aware that it has been the custom to regard 

 this well-known fact as due to a series of causes of quite different 

 kinds, the action of which may or may not co-operate in bringing 

 about the variations in the quantities of bacteria per unit of volume in 

 such a river as the Thames, but I submit to you whether, in view of 

 the new facts I have elicited, it is not at least highly probable 

 that the increased intensity of the light, acting during a longer period 

 through the summer days, is a very powerful agent in keeping down 

 the numbers of these bacterial and fungoid organisms in the river. 



It must be remembered that in my experiments, although I get 

 such pronounced positive results wherever I employ water enclosed in 

 glass vessels as the screen, I certainly do not get anything like the 

 effects that the same exposures give if I dispense with the glass of the 

 containing vessels, mirrors, &c. ; for even a thin plate of glass is a 

 decided obstruction to the rays at the violet end of the spectrum, and 

 so I have to record positive results considerably below what I should 

 get if the light from the sun and blue sky reached the organisms 

 through the medium of water only. In the experiments I am now 

 continuing, no glass whatever is employed, and the light is not 

 allowed to traverse or be reflected from any but quartz or metal 

 surfaces. 



Unfortunately, we are as yet but scantily informed as to how far 

 the blue-violet rays penetrate into deep water, but experiments by 

 Regnard, Fol, Forel and others have shown that they do penetrate 

 to a considerable depth. Fol, for instance, showed that some light 



