ACTION OF LIGHT ON MICRO-ORGANISMS 393 



apparently lost any of their bactericidal effect on ty- 

 phoid, cholera, and some other bacilli on which he ex- 

 perimented. In some of our own experiments we have 

 found that the sun's rays after passage through a few 

 inches of water were distinctly feebler in their action 

 on anthrax spores than when this impediment was not 

 interposed. Procacci has also shown that the solar 

 rays are largely, if not entirely, deprived of their bac- 

 tericidal effect by passing through a stratum of water 

 sixty centimetres in depth. 



In its special connection with the bacteriology of 

 water we must, therefore, recognise in sunshine, and to 

 a slight extent also in diffused daylight, a powerful 

 bactericidal agency, but one the importance of which 

 there has been a considerable tendency to magnify and 

 exaggerate. On the one liand the experimental evidence 

 shows conclusively that pathogenic bacteria, at any 

 rate anthrax spores, can resist insolation prolonged 

 over many hours and even under the most favourable 

 circumstances, whilst on the other hand it is sufficiently 

 obvious that in a climate like our own a great deal of 

 the surface-water is never exposed to adequate insola- 

 tion at all, even in the case of shallow streams in which 

 under more favourable climatic conditions this bacteri- 

 cidal agency might be highly effective. Thus, whilst 

 every opportunity should be afforded for insolation in 

 the construction of waterworks, undue reliance must 

 not be placed on this any more than on any other par- 

 ticular bactericidal agency. 



