ACTION OF LIGHT ON MICRO-ORGANISMS 335 



they suggest, very prejudicially affects the bacterial 

 protoplasm itself ; for they also found that if spores were 

 exposed to light in a vacuum their power of germina- 

 tion was not impaired, and further that the nutritive 

 value of the culture-liquids which they employed was 

 not injured by insolation. Again, they showed that 

 insolation can destroy bacterial life in the entire absence 

 of culture-material altogether. These investigators, 

 moreover, extended their researches to the action of 

 light on soluble ferments, and showed that the invertase 

 of yeast is destroyed by insolation in the presence of 

 air, but not in vacuo. Finally, they found in a number 

 of very striking experiments that bacteria are much 

 more resistant to insolation when immersed in water 

 than when surrounded by any other medium, a result 

 which is obviously of great practical importance, and 

 to which we shall have to refer again. 



Tyndall ^ was led to test these remarkable investiga- 

 tions by taking flasks containing sterile solutions of cu- 

 cumbers and mangel-wurzel to Switzerland, where he in- 

 fected them with air on the Alps ; but although exposed 

 during varying periods of time to sunlight, no difference 

 in the behaviour of the organisms could be detected, 

 in all cases growths making their appearance. In the 

 following year Tyndall ^ again made some experiments 

 on the Alps, this time, however, using more nourishing 

 culture media, and whilst some flasks were infected with 

 putrefying animal and vegetable infusions, into others 

 water from a stream was introduced. In these investi- 

 gations it was found that the sun's rays had a decidedly 

 damaging effect upon the bacteria present, but that 



^ 'Note on the Influence Exercised by Light on Organic Infusions,' 

 Proc. Boy. Soc. vol. xxviii., 1878. 



^ ' On the Arrestation of Infusorial Life,' Nature^ Sept. 15, 1881, vol. 

 xxiv. p. 466. 



