The Influence of Light on Bacteria. 3 



rashly, to conclude that the destructive influence was exerted, 

 not by direct sunlight per se, but by the elevated tempera- 

 ture accompanying it. This conclusion seemed all the more 

 reasonable, since degrees of temperature were actually 

 attained, which are known, if continued long enough, to be 

 destructive to the Bacterwm termo, the organism under 

 investigation. Whether my interpretation of the nature of 

 the injurious influence at work was a correct one or not, it 

 was certainly shown by my later experiments, (Exps. VI. and 

 VII., Transactions Boy. Soc. Vict. 1882, p. 120), that expo- 

 sure to the sun's rays, for several days continuously, need not 

 destroy, or even apparently retard the development of, bac- 

 teria in a perfectly transparent nutritive solution. As a 

 matter of fact, development in one case (Exp. VI.) went on 

 most rapidly in the one of three bottles, which had been 

 exposed continuously for the longest time. If variation of 

 temperature was not the determining cause of the different 

 reaction shown by these three samples of bacterialised solu- 

 tion, then I know not how to explain that difference. 



Dr. Downes, however, not being satisfied with my criticism 

 of the conclusions arrived at by himself and Mr. Blunt, has 

 forwarded to this Society the short communication just read. 

 With reference to that communication, I must first say that 

 the suggestion offered that I could not have seen the text of 

 the papers in the Proceedings of the Royal Society is not 

 correct ; and the exactness of my references and quotations 

 ought to have shown that I had read them. With the argu- 

 ments used to show that my conclusions "were not well 

 founded, and that theirs were not open to criticism, I need 

 not take up much time. I have found, in agreement with 

 Dr. Downes, that an inoculated solution, exposed to light 

 coming through red glass, becomes turbid sooner than a 

 similar solution cultivated under yellow glass, and that it 

 may remain long transparent under exposure to light reach- 

 ing it through blue glass ; but it does not seem to me of 

 necessity to follow, that the mixed rays in white light, 

 even of great intensity, must be destructive. I have also 

 tested the comparative temperature of solutions, in 

 bottles cased -in tinfoil and naked, and have not 

 found it uniformly higher in the former, when both are 

 exposed to the sun. I can easily understand, in fact, that 

 bottles or test-tubes, wrapped all over in foil or any other 

 covering, and standing on a hot surface, such as a windowsill 

 on which the sun's rays strike, may be better protected by 



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