EFFECT OF EXTERNAL CONDITIONS ON BACTERIA 157 



solidified, the under side of the Petri dish is partially covered 

 with some opaque substance, as black paper. This may be 

 put on in the form of characters, as, for instance, the name 

 of the microorganism used or the year. The Petri dish is then 

 exposed to the action of direct sunlight for some time and is 

 then incubated. If the exposure is sufficient, those parts that 

 have been directly exposed will be clear, while a growth will 

 appear on the protected parts. This action is well 

 illustrated in the accompanying figure. In this experiment it 

 is necessary for the sunlight to pass through the glass, and in 

 doing so a great deal of the energy is lost. The work of 

 Weinzirl w r as of the greatest importance in this connection, for 

 he showed that it was possible to expose the bacteria without 

 the interposed glass directly to the rays of the sun, and that 

 when this was done the time required to kill the bacteria w r as 

 very much less than had been formerly supposed. The action 

 of the sunlight in killing the bacteria is remarkable, and fre- 

 quently an exposure of only a few minutes, five or ten, is 

 sufficient to kill all of the germs. As indicated above, this 

 germicidal action of the sunlight on bacteria is a matter of 

 great hygienic importance, and the life of the disease-producing 

 bacteria outside of the body is rendered very difficult where it 

 is possible for the direct sunlight to get at them. Undoubtedly 

 the destruction of germs takes place in great numbers in na- 

 ture, particularly in transparent substances, as water. Be- 

 cause of the germicidal action of sunlight, bacteria, in natural 

 waters, are less abundant in the upper layers than at lower 



