358 MICRO-ORGANISMS IN WATER 



In the tubes exposed to direct sunlight for from two to- 

 three hours, and to the electric light for six hours, in 

 both cases passed through a solution of alum, only a 

 slight growth was observed ; whilst in the tubes ex- 

 posed direct to the sun's rays and the electric light 

 for from three to four and six hours respectively the 

 least growth of all was observed. Thus the most dele- 

 terious effect was produced when the action of light 

 was accompanied by heat, an observation which en- 

 tirely accords with Santori's experience as regards- 

 the effect of temperature. Experiments were made to 

 ascertain which rays were the responsible agents in the 

 action of light, and for this purpose the rays of the 

 spectrum were split up by means of a prism, and the 

 gelatine tubes so arranged that only particular rays fell 

 upon them. The exposure in the rays from the electric- 

 light spectrum was continued over a period of from 

 one to three to six hours, whilst two and a half hours' 

 exposure was employed in the case of the solar 

 spectrum. 



Both series of experiments yielded the same result — 

 viz., that the typhoid bacilli grew best in the red rays ; 

 in fact, no difference could be detected between their 

 growth in these rays and in the dark cupboard. The 

 development became more and more scanty in pass- 

 ing from the red to the yellow, green, and violet rays,, 

 whilst the least growth of all was observed in the ultra 

 violet rays. 



With reference to the electric light rays, it was 

 found that one hour's exposure produced no sensible 

 effect upon the growth, and that it was only after three 

 hours' exposure that an appreciable result was ob- 

 tained. 



Geisler further made some experiments to ascertain 

 if the culture medium was chemically changed by ex- 



