iv ENVIRONMENT AND LIFE 183 



siderable influence exerted on micro-organisms by 

 very dilute reagents. Very minute doses may kill, 

 in more or less time, most bacteria, and this pro- 

 vides a basis for the prevention and treatment of 

 many diseases due to pathogenetic organisms, and 

 for the disinfection of places where pathogenetic 

 organisms are supposed to exist. It is needless 

 here to quote instances. But this investigation has 

 led to some interesting results, in showing also that 

 different organisms require different quantities or 

 proportions of the same substance to cause death, and 

 that even the same organisms, under different forms, 

 require different proportions. Here again, in these 

 very minute and elementary organisms, we find 

 physiological variability or variation in play ; here 

 again, in these minute cells, where function would 

 at first glance appear very elementary, great differences 

 exist in reaction towards external agents, and hence 

 in intimate physiology. We know, for instance, that 

 while bacteria are killed, in one species, at 80 or 90 

 centigrade, the spores of the same species require 100 

 or 120; that one species is killed at 40 or 50, 

 another at 70, 80, or 100 ; that the one thrives well 

 in such and such a culture, while the other requires 

 very different media. Here also, once more, physio- 

 logical variability is in action ; the bacillus of tuber- 

 culosis, for instance, thrives in bouillon of herring with 



