The Influence of Light on Bacteria. 5 



being very hot. The 13th was cool and cloudy, the 14th 

 bright and warm; and on the 15th, which was also bright 

 and very hot, the solution in the bottle kept inside was 

 already opalescent in the morning, the wrapped suspended 

 one likewise opalescent later in the day, both rapidly becom- 

 ing quite milky. The other three were still transparent. 

 On 2nd March both of the exposed suspended bottles began 

 to show a slight milkiness, which by the 8th had increased 

 to complete opacity. Even at this last date the one left 

 standing on the windowsill uncovered was still quite trans- 

 parent. The general results of this mixed experiment were 

 — first, that a solution exposed to diffused light, and even to 

 some extent to the direct rays of the sun, developed bacteria 

 as quickly as that contained in a bottle carefully wrapped 

 in paper ; and, secondly, that bottles suspended in the sun 

 showed full development of bacteria, though at a later date, 

 while one which had been standing on a hot window sill 

 continued to be quite transparent. The amount of light 

 was not greater in the latter case, but the temperature 

 attained in the sun was considerably higher : and I cannot 

 think of anything but this difference of temperature which 

 could have brought about the different results. The actual 

 difference in the temperature of the solutions, in bottles 

 standing and suspended, is very considerable, since I found 

 that, with the thermometer at about 118 degs. Fahr. in the 

 sun, fluid in the bottom -of a bottle, standing on a window- 

 sill beside it, rose readily to 108 degs. Fahr.; while fluid in 

 suspended bottles, whether naked or covered with tinfoil, 

 rose only to 98-102 degs. Fahr., when the thermometer 

 marked as much as 125-132 degs. Fahr. 



The difficulty I have experienced in carrying out com- 

 parative tests lay in preserving uniformity of temperature, 

 with varying intensity of solar light. I tried first to get 

 over the difficulty in the following way: — 



Exp. II. — Two bottles, each containing two drams of in- 

 oculated solution, were suspended inside but just behind 

 the glass of a high window, on which the sun fell nearly all 

 day. One was wrapped in paper, the other exposed. This 

 was on 19th February at two p.m., the day being bright but 

 cool. The 20th was cloudy in the afternoon, the 21st bright 

 and warm, and on the 22nd the solution in both was dis- 

 tinctly opalescent, though most markedly so in the covered 

 one. On the 24th both were quite milky, but still the 

 bacterial growth was most marked in the wrapped bottle. 



