260 Professor Ward [April 27, 



like the hygienic importance they really have. But it has required 

 much careful experimental investigation to decide this matter, and I 

 propose to show you some of the principal steps in the chain of 

 proof. 



The fact that bacteria, which would multiply at an enormous rate 

 in certain organic liquids, as, for instance, dilute meat broth, cease 

 more or less evidently to do so if the fluid containing them is 

 exposed to intense sunshine, was first pointed out by Messrs. 

 Downes and Blunt in 1877 and 1878, and further experiments 

 have abundantly confirmed their result. The establishment of this 

 truth led to a more or less desultory controversy, turning on the 

 question whether the effect was merely one of relatively high 

 temperature, or whether it was really due to the light rays — in 

 other words, whether those rays of the solar spectrum known to us 

 especially by their thermal actions, induced the death of the inso- 

 lated bacteria, or whether other rays of the solar spectrum were 

 concerned. 



This controversy was brought virtually to an end by the various 

 experiments of Downes and Blunt, Arloing, Janowsky, and others, 

 who showed that in many cases at any rate the bacteria in the tubes 

 of broth, &c, were either killed outright, or rendered almost inca- 

 pable of further development in ordinary daylight, or in tubes cooled 

 by ice, or — in later experiments — in tubes behind screens which cut 

 off the heat-rays to such an extent that no question of temperature 

 could possibly be raised. 



Meanwhile, another controversy arose, and was also carried on in 

 a more or less disjointed manner through a series of years. This 

 turned on the question — granted that the bactericidal effect is not 

 due to high temperatures, may it not be merely the result of some 

 poisoning effect of certain substances produced in the food-medium 

 (broth, &c.) under the oxidising effect of the air in the illuminated 

 tubes. 



Duclaux was, I believe, the first to assert this definitely, and it 

 was regarded by many as confirmed by experiments performed by 

 Boux in 1887. It was pointed out that certain rays — more especially 

 those towards the violet end of the spectrum, and known popularly as 

 the "chemical" rays — are very apt to promote oxidative decomposi- 

 tions in solutions of organic substances, such as the broth used for 

 cultivating these bacteria. It was also shown that the death of the 

 bacteria in question did not result if all traces of oxygen are 

 removed from the tubes previous to insolation. It had also been 

 rendered at least highly probable — though there was as yet no agree- 

 ment on this point — that certain rays of light only, some thought 

 the red, others the blue-violet rays, are especially concerned in the 

 process. The conclusion drawn, therefore, was that the effect was 

 primarily due to a poisoning action of the broth or other food-material 

 due to the energetic oxidations promoted in it by the solar rays when 

 exposed to the light of the sun. 



