THEIR MODE OF ACTION. 399 



views of seas, fertile fields, and luscious fruits, but 

 leave us languishing when we have most need of 

 what they promise. 



It is certain, that the action of contagions is the 

 result of a peculiar influence dependent on chemical 

 forces, and in no way connected with the vital prin- 

 ciple. This influence is destroyed by chemical ac- 

 tions, and manifests itself wherever it is not sub- 

 dued by some antagonist power. Its existence is 

 recognised in a connected series of changes and 

 transformations, in which it causes all substances 

 capable of undergoing similar changes to participate. 



An animal substance in the act of decomposition, 

 or a substance generated from the component parts 

 of a living body by disease, communicates its own 

 condition to all parts of the system capable of enter- 

 ing into the same state, if no cause exist in these 

 parts by which the change is counteracted or de- 

 stroyed. 



Disease is excited by contagion. 



The transformations produced by the disease as- 

 sume a series of forms. 



In order to obtain a clear conception of these 

 transformations, we may consider the changes which 

 substances, more simply composed than the living 

 body, suff*er from the influence of similar causes. 

 When putrefying blood or yeast in the act of trans- 

 formation is placed in contact with a solution of 

 sugar, the elements of the latter substance are trans- 

 posed, so as to form alcohol and carbonic acid. 



A piece of the rennet-stomach of a calf in a state 

 of decomposition occasions the elements of sugar to 

 assume a different arrangement. The sugar is con- 

 verted into lactic acid without the addition or loss 

 of any element. (1 atom of sugar of grapes C12 

 H12 012 yields two atoms of lactic acid =2 (C6 

 H6 06.) 



When the juice of onions or of beet-root is made 

 to ferment at high temperatures, lactic acid, mannite, 

 and gum are formed. Thus, according to the differ- 



