236 Progress of Rational Pathology. 



weakly, that every mechanical alteration by warmth, friction 

 and contact with indifferent bodies, causes such a distur- 

 bance among their molecules, that they separate, and 

 then form anew, fresh and more natural combinations. 

 Substances, when in motion and in the act of transposing 

 their molecules, are supposed to communicate their mo- 

 tion to other substances, just as ferment or putrefying mat- 

 ter does to sugar, the elements of which then form more 

 simple combinations, such as alcohol and carbonic acid. Al- 

 though Liebig now tries to add new positive weight to this 

 theory, it is in fact a repetition of the old doctrine, that 

 the spreading of fermentation and of contagion are analogous 

 processes. On sufficient consideration this theory has been 

 found utterly untenable— as little tenable is Naumann's 

 chemical theory of contagion. Winther tries to make out 

 ammonia to be the actual matter of contagion, while Liebig 

 regards that substance as only the medium of the gaseous 

 form of contagion. But all chemical theories have a radical 

 defect : they are not even hypotheses ; they do not explain 

 facts, but hidden conjectural phenomena, and lay down the 

 necessity of the operation of causes, which after all may not 

 operate at all. It is indeed often assumed, but it never has 

 been proved, that contagion always reaches the blood : it 

 is just as uncertain that it is afterwards thrown out of it, as 

 it is certain that pustules, papulae, &c. are not the secreting 

 organs of contagion, but the result of inflammation of the 

 skin. A theory of contagion, that makes pretension to the 

 character of a philosophic hypothesis, must begin not by 

 assuming the mutual action of infectious matter and blood, 

 but with the phenomena of contagions and contagious 

 diseases, that is with the operations of contagious matter, 

 which are real and appreciable by our senses, and this method 

 conducts us to the other theory. 



Parasitic theory. — Holland looks for the parasites in the 

 animal kingdom (especially among insects), and regards ill- 



