Influence of Light on Development of Bacteria. 123 



favourable to the development of life than that of the shaded 

 ones." 



When it is considered how much greater is the difficulty 

 of destroying bacteria or their germs in the dry than in the 

 moist state, either by heat or disinfectants, it might almost 

 with safety be concluded that insolation, which fails to 

 destroy the bacterium termo in solutions, is not likely to 

 injure it when dried. 



As reported in my previous communication to this Society, 

 I found dried bacteria resist a temperature of about 212° F. 

 for fifteen minutes, and, therefore, no solar heat could be 

 expected to kill fchem. But as desiccation, when sufficiently 

 complete, has that effect, it might readily happen that 

 exposure to the sun's rays in hot weather might act destruc- 

 tively, in virtue of its drying effect. To test the influence of 

 insolation on the dry bacteria, I soaked blotting-paper with 

 bacterialised solution, obtained from a bottle used in one of 

 the previous experiments, and exposed it to the sun freely 

 suspended by a piece of thread. Similar pieces of paper 

 were hung up in a shady but well-aired passage, and in a 

 well-lighted room. This was done twice ; and, to test the 

 condition of the bacteria in the pores of the paper, the fol- 

 lowing precautions were taken : — Bottles, as before, after 

 receiving about two drams of pure solution, were plugged 

 with cotton wadding, and then kept for some time in boiling 

 water to secure complete sterilization. After time was 

 allowed for cooling, the plug was taken out, a little square 

 of the blotting-paper dropped quickly in, contact only 

 with scissors being allowed, and the plug replaced. In the 

 first series of experiments, carried on in hot weather, it was 

 found that, after two days, the bacteria had not been killed 

 in any of the papers ; that, after four days, they had been 

 killed in that exposed to the sun, and that hung in a current 

 of air, but in the shade ; and not killed in that which had 

 been suspended in bright, diffused light. After seven days, 

 the last also failed to bring about milkiness in the solution. 

 I conclude, therefore, that it was simply a question of desic- 

 cation with all of them, the time needed to produce destruc- 

 tion in that way varying with temperature and exposure to 

 currents of air. In the other series, a similar result was 

 reached. The growth of bacteria in the bottle containing 

 the sun-dried paper was later in occurring than in the others, 

 but was not completely prevented even after five days 

 of exposure. The interest of these experiments consists in 



