162 AYERS AND JOHNSON: DESTRUCTION OF BACTERIA 



When milk was exposed in this apparatus, very satisfactory 

 bacterial reductions were obtained as shown in the above table. 



Thruout the experiments the temperature of the milk during 

 exposure never was over 30°C. (86°F). Consequently high tem- 

 perature played no part in the destruction of the bacteria. 



Numerous experiments were performed to determine the action 

 of the rays on vegetative cells and spores and to study numerous 

 conditions which affect the power of the rays. A few experi- 

 ments were also made to determine the possibiUty of sterilizing 

 milk bottles by the rays. The results can be best presented in 

 the form of a summary as follows: 



1. When milk was exposed in thin layers to ultraviolet rays 

 there was a marked reduction in the bacterial content. 



2. The action of the rays was entirely independent of the action 

 of heat since the temperature of the exposed milk was never over 

 30°C. (86°F.) 



3. The most satisfactory method of exposure was over two 

 revolving drums the tops of which were at a distance of 4 inches 

 below the light tube of the lamp. 



4. The two factors of greatest importance in the successful 

 application of the rays were the thickness of the layer and the 

 length of exposure. A thin layer allows a more complete pene- 

 tration of the rays and the longer the exposure the more chance 

 they have to act. 



5. Ultraviolet rays exerted a greater bactericidal action on 

 vegetative cells in milk than on spores when exposed under the 

 same conditions. 



6. No greater action of the rays on bacteria was observed when 

 the bacteria were weakened by pasteurization immediately pre- 

 ceding the exposure. 



7. From the study of two samples of milk exposed to ultra- 

 violet rays it was apparent that the rays did not exert any specific 

 bactericidal power on any particular group of bacteria in the milk. 

 As stated before, however, there was a difference in the action of 

 the rays on bacteria in the vegetative and spore state. 



8. Under similar conditions of exposure there seemed to be 

 somewhat less bacterial reduction in a 15 per cent cream than in 



