376 POISONS, CONTAGIONS, MIASMS. 



composition in the blood may affect individual constituents only 

 of that fluid, which will become destroyed and disappear, whilst 

 its other parts will maintain the original form. 



Several kinds of contagion are propagated through the air : so 

 that according to the view already mentioned, we must ascribe 

 life to a gas, that is, to an aeriform body. 



All the supposed proofs of^the vitality of contagions are 

 merely ideas and figurative representations, fitted to render the 

 phenomena more easy of apprehension by our senses, without 

 explaining them. These figurative expressions, with which we 

 are so willingly and easily satisfied in all sciences, are the foes 

 of all inquiries into the mysteries of jiature ; they are like the 

 fata morgana, which show us deceitful 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 pe- 

 culiar influence dependent on chemical forces, and in no way 

 connected with the vital principle. This influence is destroyed 

 by chemical actions, 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 sub- 

 stance generated from the component parts of a living body by 

 disease, communicates its own condition to all parts of the system 

 capable of entering into the same state, if no cause exist in 

 these parts by which the change is counteracted or destroyed. 



Disease is thus excited by contagion. 



The transformations produced by the disease assume 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, suffer from the influence of simi- 

 lar causes. When putrefying blood or yeast in the act of trans- 

 formation is placed in contact with a solution of sugar, the ele- 

 ments of the latter substance are transposed, so as to form alcohol 

 and carbonic acid. 



