24 BACTERIA 



60-70° C. These latter are termed thermophilic bacteria. 

 The average thermal death-point is at or about 50° C. 



Light acts as an inhibitory or even germicidal agent. This 

 fact was first established by Downes and Blunt in a memoir 



-T' 



Inoculating Needles 



Plantinum wire fused into glass handles 



to the Royal Society in 1877. They found by exposing 

 cultures to different degrees of sunlight that thus the growth 

 of the culture was partially or entirely prevented, being most 

 damaged by the direct rays of the sun, although diffuse day- 

 light acted prejudicially. Further, these same investigators 

 proved that of the rays of the spectrum which acted inimi- 

 cally the blue and violet rays acted most bactericidally, next 

 to the blue being the red and orange-red rays. The action 

 of light, they explain, is due to the gradual oxidation which 

 is induced by the sun's rays in the presence of oxygen. 

 Duclaux, who worked at this question at a later date, con- 

 cluded that the degree of resistance to the bactericidal in- 

 fluence of light which some bacteria possess might be due 

 to difference in species, difference in culture media, and 

 difference in the degrees of intensity of light. Tyndall tested 

 the growth of organisms in flasks exposed to air and light 

 on the Alps, and found that sunlight inhibited the growth 

 temporarily. A large number of experimenters in Europe 

 and England have worked at this fascinating subject since 



