LABORATORY RESEARCH, ETC. 269 



Attenuation of Pathogenic Microbes. — Successive 

 cultures have established, as we have seen, the pos- 

 sibility of attenuating virus, and transforming it into 

 vaccine. The processes employed to attain this object 

 are complex and varied, according to the species of 

 bacterium with which we have to do. 



Thus, for fowl-cholera, Pasteur found that cultures 

 dating from fifteen days, or from one, two, eight, and 

 ten months, progressively lost their virulence, and he 

 believes this attenuation to be due to the action of the 

 oxygen of the air. So, again, Koch supposes that the 

 action of the air and the desiccation of the germs 

 produces, after a time, the natural extinction of the 

 disease. 



Toussiant and Chauveau attenuate the virus of 

 anthrax, as we have seen, by subjecting it to a tem- 

 perature of from 42° to 4.3°. Pasteur and Thuillier have 

 attenuated the virus of swine fever by passing it 

 through the system of a rabbit. Pasteur has also 

 attenuated the virus of rabies, of which the microbe 

 is still unknown, by passing it successively through 

 the systems of a rabbit, monkey, etc. 



Finally, the same result may be obtained by add- 

 ing various antiseptic substances to culture liquids, 

 and thus weakening the virulent action of the 

 microbe. 



Vaccination and Inoculation. — The attenuated 

 virus or vaccine thus obtained may be used for inocu- 

 lation in quantities which experience indicates to 



