288 BULLETIN No. 161 [November, 



oxygen than when the oxygen is excluded. He thinks that from 

 the nature of the spore constituents more oxidation products are 

 formed in the spore than in the vegetative cell, and that for this 

 reason the spore is more readily killed. Dieudonne 31 (1894) con- 

 cludes that the formation of hydrogen peroxid by the action of 

 sunlight on the presence of oxygen forms an important part in the 

 killing of bacteria. Kruse 76 (1895) and Richardson 118 (1893) 

 also came to this same conclusion. Thiele and Wolf 147 (1906) 

 carried out carefully planned experiments in which their cultures 

 of B. prodigiosus, B. pyocyaneus and B. coll were exposed in the 

 presence of air, oxygen, or hydrogen. They took special precau- 

 tions to confine the gases used, with mercury joints, so as not to 

 allow the slightest diffusion. They found that the bacteria were 

 killed as readily anaerobically as aerobically, and conclude, there- 

 fore, that the killing of the bacteria by light is not assignable to 

 the indirect influence of oxidation of water. They exposed their 

 cultures in broth diluted i-iooo, physiological salt solution and 

 Elbe river water. 



It can readily be shown that by the exposure of physiological 

 salt solution to sunlight for the length of time the investigators ex- 

 posed their cultures there is formed some condition sufficient to 

 kill bacteria. The question whether the bactericidal action is due 

 to an injurious chemical formed by the action of the sunlight, or 

 whether it is a direct bactericidal action of the light, is still unset- 

 tled. The facts thus far established indicate that the action is due 

 to both a chemical and a direct bactericidal action, sometimes one 

 and sometimes the other being predominant, depending upon the 

 condition of the experiment. A more important fact, which has 

 been especially emphasized in a recent publication (see Weinzirl, 

 Table 2) is that bacteria when directly exposed to sunlight are 

 killed in a few minutes, in contrast to most of the published results 

 of a few hours to a few days. This is due to the well-known 

 germicidal effect of the ultra violet rays. Since glass is an excel- 

 lent screen for these rays, an exposure of bacteria in glass contain- 

 ers lengthens very greatly the time necessary to kill. 



Powerful beams of ultra violet rays, are now artificially pro- 

 duced with the quartz mercury vapor lamp. A number of these 

 machines are upon the market. The rays from them are so effec- 

 tive that bacteria directly exposed in thin layers are killed almost 

 instantly. Indeed, so strong are the most powerful of these lamps 

 that the skin accidently exposed is killed and later sloughs off. 



It is stated that the sterilization of a city water supply can be 

 done economically with these lamps. A test 5 at Marseilles, France, 

 shows that the cost per million gallons of water is only ten dollars, 



