Influence of Light on Development of Bacteria. 121 



that the warmth (98° F.) outside, on the 10th and 11th, 

 favoured that development in the bottle exposed to it ; the 

 others, inside of the house, being at a lower temperature. 

 Long and continuous insolation had here certainly been 

 little, if at all, inimical to the growth of bacteria. 



Exp. VII. On 14th April, at 12.30 p.m., I inoculated six 

 drams of solution with two drops of bacterialised fluid, and 

 divided it equally over three bottles. They were all 

 suspended in the sun, one of them having been first wrapped 

 in brown paper. The weather was cloudy and almost cold 

 on the following days, the 19th and 20th, however, being 

 bright all day; and only on the 21st were the exposed 

 bottles found to be opalescent. The solution in the covered 

 bottle was quite milky. My interpretation of these con- 

 ditions was, that the coldness of the weather had checked the 

 multiplication of the bacteria in the first days, growth only 

 beginning actively in the brighter and warmer weather of 

 the 19th and 20th. The more advanced development in the 

 covered bottle was most naturally to be ascribed, I think, 

 to the wrapping keeping it at a more uniform temperature, 

 and especially preventing that from sinking so low during 

 the night. 



The result of these two experiments was clearly to show 

 that insolation, associated with moderate or low temperature, 

 has no destructive influence on bacteria, not even apparently 

 retarding their growth. I was, therefore, driven to con- 

 clusions directly contradictory to those both of Professor 

 Tyndall and of Messrs. Downes and Blunt. The doubt, of 

 course, which at once suggested itself was, whether the sun's 

 rays, even in summer in England, would raise a solution 

 exposed to them to a temperature sufficient of itself to 

 destroy bacteria. To settle this point it was necessary, first 

 of all, to ascertain the lowest temperature at which the 

 Bacterium iermo is paralysed or killed. This information 

 has been provided by the careful experiments of Dr. Eduard 

 Eidam, reported in Cohn s " Beitrage Zur Biologie der 

 Pnanzen" (heft, in., p. 208). He found that while very low 

 temperatures check indefinitely the growth of this organism, 

 growth becomes more active with gradual elevation up to 

 35° C. (95° F.). Temperatures above this are again less 

 favourable, and between 40° and 45° C. (104°-113° F.), 

 the bacteria remain in a torpid condition, a kind of heat 

 rigidity (Warmestarre), but are not killed. An exposure for 

 seven days to a temperature of 45° C. w T as sufficent to cause 



