3] EFFECT UPON GENERAL FUNCTIONS 173 



spectrum (obtained by the use of a quartz prism) was em- 

 ployed, a bactericidal effect was obtained (provided no glass 

 intervened) in the ultra-violet. That the action of the light 

 was not in these cases primarily upon the food-film was shown 

 by the fact that a plate of sterile agar, exposed behind a stencil 

 plate, and then laid flat on a film of dried unexposed spores, 

 permitted the uniform growth of the spores, in the illumined as 



FIG. 48. Plate of anthrax spores, exposed for 5 hours to the solar spectrum in 

 August, ami incubated for 48 hours. The horizontal line shows the length of 

 the spectrum. The vertical lines are not FRAUENHOFER'S lines, but serve to 

 show the limits of the principal regions of the spectrum. The clearest area is 

 that where fewest spores have developed in the incubation where, consequently, 

 the bactericidal effect was greatest. (From WARD, '9i.) 



well as in the unillumined region. All these observations show 

 that the bactericidal action of light is due to the action of the 

 chemical rays on the protoplasm. 



Another fact of importance, first discovered by DOWNES and 

 BLUNT, is that light has no effect upon bacteria when they are 

 in a vacuum. This abundantly confirmed observation indicates 

 that death only secondarily results from light. The primar} r 

 cause of death is an oxidation process, a process rendered 

 possible by the mediation of light. As we have seen (p. 162), 

 many organic compounds undergo oxidation in the highly 

 refracted light rays. Probably there are in bacteria such com- 



