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CHAPTEE IX. 



THE ACTION OF LIGHT ON MICRO-ORGANISMS IN WATER 

 AND CULTURE MEDIA 



THE above subject, although at first sight somewhat 

 outside the application of bacteriological science which 

 we have so far been considering, is one of such immense 

 importance in connection with the vital phenomena of 

 bacteria, and is indissolubly associated with so many 

 of the problems with which the investigator is con- 

 fronted, that a knowledge of what has been already 

 achieved in this direction is essential to those who 

 purpose taking up the study of micro-organisms in 

 water. Indeed, some of the more recent * experiments 

 have a direct bearing upon the questions we have been 

 discussing, and whilst, therefore, giving a general survey 

 of the researches on light which have so far been made, 

 a more particular account will be found of those in- 

 vestigations which are concerned with the behaviour of 

 micro-organisms in water when exposed to light. 



To Downes and Blunt 1 belongs the credit, not only 

 of having first demonstrated the bactericidal effect of 

 light in a memoir published in the Proceedings of the 

 Royal Society in 1877, but in having at once so perfectly 

 indicated almost all the factors connected with this 

 action that the further investigations which have been 

 carried out on this subject during the past fifteen years 



1 ' Researches on the Effect of Light upon Bacteria and other Or- 

 ganisms,' Prop. Hoy. Soc. vol. xxvi. No. 184, 1877, Dec. 6. 



