EFFECT OF ULTRA-VIOLET RAYS. 



389 



Table II. — Experiment 2: Agar plates inoculated with amcebce aiid cultivated for 

 2Jt hours before exposure to ultra-violet rays. Plates contained encysted forms. 



Distance from 

 lamp. 



No. 



Time of exposure in seconds. 



Con- 

 trol, 

 not ex- 

 posed. 



10 



20 



30 



40 



50 



A. 10 cm. 



1 

 2 

 3 



4 



+ 





+ 



— 



— 



+ 

 + 

 + 



B. 15 cm. 



5 

 6 



7 

 8 



+ 



+ 



+ 



— 



— 



+ 

 + 

 + 



C. 23 cm. 



9 



10 

 11 

 12 



+ 

 + 

 + 



+ 



+ 

 + 

 -1- 



-1- 

 -t- 

 + 

 + 



+ + + -1- 



+ 

 + 

 + 

 + 



+ 

 + 



By consulting this table it becomes apparent that exposures at a 

 distance of 83 centimeters from the lamp were entirely ineffective, but 

 that the amcebfe exposed at distances of 10 and 15 centimeters were 

 invariably killed in forty seconds, and usually killed in ten seconds, 

 although an occasional positive result was obtained even after exposures 

 of thirty seconds. However, these failures may readily be explained. 

 The growth of amoebae and associated bacteria on plates at the end of 

 twenty-four hours is dense, forming a very perceptible layer on the 

 surface of the agar. It is well known that the ultra-violet rays of light 

 have no greater power of penetration. Therefore some of the amoebae 

 in a few of the exposures have survived owing to the absorption of the 

 rays by the superficial layers of the growth. This could not occur in 

 water passing through a sterilizer operated by ultra-violet rays. 



Since all of these plates contained many encysted amoebae, and since 

 positive cultures could never be obtained from those plates exposed 

 forty seconds or more at 10 and 15 centimeters, and rarely after ten- 

 second exposures, we are justified in concluding from this experiment 

 that encysted amoebae are killed on solid media by ultra-violet radiation. 

 The subject of encysted amoebae will be further discussed in Experiment 

 7, where a fluid medium^ was used. 



The effect of ultra-violet rays upon amoebae in a fluid medium was now 

 tested. These experiments are considered to be of much more value 

 than the preceding ones, because they approximated the conditions which 



