24 THE GERM THEORY OF DISEASE. 



He thus recognized a gaseous form of ferments. 



We have already spoken of the fact that when fermentation 

 spontaneously ceased, it could not again be set up ; and its 

 application to those contagious diseases which can, ordinarily, 

 be had but once by the same individual. This theory was 

 elaborated anew by Liebig, with telling effect against the vital 

 theory. It was considered that in must or wort there is a 

 certain substance that is decomposed by the peculiar molecular 

 motion of vinous fermentation, namely, sugar; while the other 

 portions of the compound are not necessarily affected. The 

 blood is a very complex substance, containing many compounds 

 held together by feeble molecular affinities, any one of which 

 may be decomposed. In case of the contagious diseases, the 

 decomposing body which causes the particular disease acts 

 upon some single compound of the blood, decomposing it. 

 If this particular component of the blood be very important 

 to the vital processes, the disease will be very grave. If, on 

 the other hand, this particular compound be not very im- 

 portant, the disease will be correspondingly light. Now in 

 the progress of the disease this particular compound is all 

 decomposed, as the sugar in grape juice, and if the patient 

 has been able to withstand the shock, he returns to health 

 with this one compound of the blood lacking, and it often 

 happens that it is replaced very slowly, if at all ; therefore, 

 the patient is protected for a time or permanently. 



Liebig explained the susceptibility of children to certain 

 diseases while adults were insusceptible, by assuming that the 

 constituent of the blood upon which the molecular motion 

 poison acted in the child was absent in the adult. He also 

 explained the differences in susceptibility of different indi- 

 viduals in the same way. 



Finally, Liebig says, "all the supposed proofs of the vitality 

 of contagions are merely ideas and figurative representations, 



