388 CHAMBERLAIN AND VEDDER. 



of the plates inoculated with, amoebse showed growth after this exposure. 

 However, it will be seen that the effect of the rays on the bacteria was 

 very pronounced even in five-second exposures, because instead of a 

 profuse growth such as was observed on the control plates, only a few 

 scattered colonies were found. If the lamp had been in preliminary 

 operation for a few seconds, so that it was producing its maximum of 

 rays at the time the exposure began, it is probable that all of the plates 

 exposed for five seconds would have been sterile. It would have been 

 very difficult however to make accurately timed exposures with the lamp 

 continuously in operation, and in any case the results are sufficient for 

 our present purpose. Others have already demonstrated that bacteria 

 are killed in five seconds by such exposures. This experiment shows 

 that in exposures close to the lamp amoebae are destroyed by the ultra- 

 violet radiations quite as readily as Bacillus typhosus or Bacillus dy- 

 senterice. 



At a distance of 23 centimeters from the lamp it required twice as 

 long an exposure to kill amoebae as compared with Bacillus typhosus 

 and Bacillus dysenteric. This is unimportant with regard to practical 

 sterilization of water in the apparatus as manufactured, for the reason 

 that in these sterilizers all the water is forced to flow close to the 

 lamp, and furthermore the lamp used is probably much more powerful 

 than the arc employed by us. 



The conclusions which can clearly be drawn from this experiment are 

 that amcebse are destroyed by exposure for a few seconds to ultra-violet 

 rays, and that at 10 centimeters distances they are as readily killed by 

 this agent as are dysentery and typhoid bacilli. 



However, it was desirable to determine whether encysted amoebae 

 would be killed by this method, since it is quite probable that amoebae 

 in a water supply would usually be present in the encysted form. The 

 following experiment was performed for this purpose. 



Experiment 2. — Plates were inoculated exactly as in the first experi- 

 ment, but were incubated for twenty-four hours before exposure. During 

 this period of growth many of the amoebae became encysted. Since it 

 is impossible to tell by microscopic examination whether an encysted 

 amoeba is dead or alive, this point was determined by cultures. After 

 exposure to the ultra-violet rays the growth was scraped from the surface 

 of the plate and inoculated into a tube containing a mixture of 1 part 

 of plain bouillon and 2 parts water. This culture was incubated for 

 twenty-four hours and then examined for motile amoebae. Cultures were 

 also made from control plates that had not been exposed to the light. 

 The results of this experiment are shown in Table II. 



