Contagious and Epizootic Diseases. 61 



XII. — IMMUNITY BY INOCULATION WITH VIEUS GROWN IN FEEE 

 CONTACT WITH AIE. 



The same principle operates in this as in the last method, 

 the bacterium or other germ living in free contact with air 

 acquires the habit of using more oxygen than it can secure 

 in the animal tissues, and when transferred to these it grows 

 in a sickly manner and is easily thrown off by the living 

 animal tissues. This is largely operative in slowly disin- 

 fecting buildings freely open to the air, infected yards, parks, 

 and other open places, while it determines that virulent 

 matters closely shut up in sewers, manure-heaps, cess-pools, 

 close areas under floors, compact, water-logged, or filth-satu- 

 rated soils, or indeed wherever the air cannot freely reach it, 

 retain their infecting qualities for a much longer time, and 

 at times, as in cholera, yellow fever, and typhoid, have 

 them materially enhanced in potency. In my experiments 

 with swine-plague and septic matters I invariably found 

 that material the most deadly which had been grown in 

 closed flasks with a very limited supply of air, while that 

 which was grown in thin layers and with free access to air 

 steadily lost in potency, and finally produced a disease so 

 mild that it could be resorted to as a means of preventing 

 losses' in herds. 



Xm. — IMMUNITY BY INOCULATION WITH VIEUS WHICH HAS 

 BEEN EXPOSED TO COMPEESSED OXYGEN. 



Tliis is based on the same principle with the last, only in 

 place of a lengthened exposure to the oxygen in the air 

 there is a tempoi'ary exposure to pure oxygen under extra 

 pressure. Chauveau has especially labored in this field, and 

 found that, by carefully graduating the pressure and the pe- 

 riod of exposure, he could secure such debility or lessened 

 potency in the germs as would determine a mild and non- 



