THEIR MODE OF ACTION. 373 



tumultuous transformation, the former in vegetable 

 juices, the latter in blood, in both of which fluids 

 respectively their constituents are contained, and 

 they are reproduced from these fluids with all their 

 characteristic properties. The precipitated yeast 

 of Bavarian beer on the other hand acts entirely 

 upon the sugar of the fermenting liquid and occa- 

 sions a very protracted decomposition of it, in 

 which the gluten which is also present takes no 

 part. But the air exercises an influence upon 

 the latter substance, and causes it to assume a 

 new form and nature, in consequence of which this 

 kind of yeast also is reproduced. 



The action of the virus of cow-pox is analogous 

 to that of the low yeast ; it communicates its own 

 state of decomposition to a matter in the blood, and 

 from a second matter is itself regenerated, but by a 

 totally different mode of decomposition ; the pro- 

 duct possesses the mild form, and all the properties 

 of the lymph of cow-pox. 



The susceptibility of infection by the virus of 

 human small-pox must cease after vaccination, for 

 the substance to the presence of which this sus- 

 ceptibility is owing has been removed from the 

 body by a peculiar process of decomposition artifi- 

 cially excited. But this substance may be again 

 generated in the same individual so that he may 

 again become liable to contagion, and a second or 

 a third vaccination will again remove the peculiar 

 substance from the system. 



Chemical actions are propagated in no organs so 



