20 



Dr. A. Downes. On the 



[Jan. 14, 



four hours at 50—52° C. (122—125-6° F.), and by one hour at 60° C. 

 (140° F.). He argued that some of these temperatures were com- 

 monly attained in the sun's rays in Australia, and even in England ; 

 he thought that 125° F. (51 - 6° C.) would occasionally be experienced 

 for a few hours, and he concluded that his, our own, and Tyndall's 

 results were all effects of temperature. 



The argument is fallacious. It is true that some organisms in 

 certain stages of their development may be destroyed by lower degrees 

 of heat than is commonly supposed. Duclaux has given an instance 

 in which forfcy-eight hours at 38° C. (100*4° F.) was fatal to some very 

 old yeast globules. 



But even of these it is true only in their vegetative forms ; their 

 spores (and the spore-form is doubtless that in which the micro- 

 organisms originally existed in our nutrient liquids) resist elevations 

 of heat far surpassing anything noted above. Were it otherwise 

 bacterial life would probably soon cease to be. It is obviously incor- 

 rect to argue that, because some organisms in some phase of their 

 existence are destroyed by moderate heafc, all organisms, in all phases 

 and under all conditions, are so too, and any inference drawn from 

 such reasoning must be rejected. 



Moreover, I have already shown that the laminated lead used in our 

 experiments absorbed radiant heat in greater degree than the bare 

 glass, and consequently that our incased tubes would be more affected 

 by solar heat than our insolated. And I need only refer to our 

 previous demonstration, that the greatest effect on micro-organisms 

 is produced by those rays which occupy the cooler portion of the 

 spectrum. 



Dr. Jamieson failed with diffused light. His failure was due to his 

 method of experimenting.* 



I have already said that an essential element of success is to appor- 

 tion the natural resistance of the cultivation liquids to the amount of 

 light available. Bacterial development once started usually outruns 

 even direct sunlight, both by increasing the opacity of the fluid, and 

 by quickly reducing the amonnt of oxygen. Naturally diffused light 

 would be far slower in action than the direct solar ray, and we must 

 select either very cool weather for the experiment, or must choose 

 solutions of considerable resistance. 



Keeping this principle in view, I have found it easy to show that 

 diffused light possesses properties differing only in degree from those 

 which we have demonstrated in regard to direct sunlight. I have 

 made a number of experiments in which ordinary thin test-tubes 

 plugged with cotton-wool were placed in a box (20*5 cm. cube) lined 



* Moreover, lie seems to have placed his bottles inside a window. The absorp- 

 tive power of glass has always prevented me from succeeding in such circum- 

 stances. 



