ACTION OF LIGHT ON MICEO-OEGANISMS 337 



in the act of spore-formation. This bacillus was 

 inoculated into both milk and broth, and from each 

 cultivation a small drop was introduced into separate 

 sterile empty flasks, in which the drops were allowed 

 to evaporate. The flasks, each of which thus contained 

 the dry residue of a drop, were then exposed to sun- 

 light for definite lengths of time, after which a small 

 quantity of sterile bouillon or some other suitable 

 culture material was added to the various flasks, the 

 latter being then placed in the incubator. After fourteen 

 days' exposure in the case of the milk-cultures the sun- 

 light had produced no effect ; after one month's insolation 

 the development of the spores had been delayed ; after 

 two months' insolation two out of four of the flasks 

 remained sterile. Spores similarly treated, but shielded 

 from light, were found capable of development even 

 after three years. On the other hand, the spores obtained 

 from the bouillon (Liebig extract) cultures were far 

 more quickly affected by exposure to sunlight than 

 those obtained from milk. Thus, after fifteen days' 

 insolation, out of three flasks one remained sterile ; after 

 one month's exposure two out of the three showed no 

 growth ; and at the end of two months all three flasks 

 were sterile. 



Duclaux therefore concludes that the degree of re- 

 sistance to the action of light possessed by bacteria 

 varies, not only according to the species, but also, in 

 one and the same species, according to the nature of 

 the culture medium from which 'they are obtained, as 

 well as the intensity of the light to which they are 

 exposed. 



In a second investigation Duclaux l used six varieties 

 of micrococci, and -took precautions to prevent the tern- 



1 ' Influencejle la Lumiere du Soleil sur la Vitalite de Micrococcus,' 

 Compt. rend., 5 Aout, 1885, tome ci. 



Z 



