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the world. Epidemics reappeared in ships and houses long 

 after the patients had been removed from them, and even 

 after they had been, as was supposed, thoroughly disinfected. 

 Men said is this not exactly what would occur if we were to 

 imagine the infecting material a seed which remains 

 dormant until it finds a suitable soil in which to grow ? This 

 view of the question obtained so much favour that out of it 

 developed what is known as the germ theory, or more 

 strictly, the theory of contagium virum. Those opposed to 

 this view objected that as nothing in the nature of a germ 

 had ever been seen, what proof is there that the contagious 

 material is not a virulent chemical poison, or, if not that, an 

 unorganised ferment, similar to pepsine for instance ? The 

 answer is, first, if it were a chemical substance, it would act 

 as such, and when its force was expended the process would 

 cease. Again, although unorganised ferments have the 

 power of causing change in other substances without under- 

 going alteration themselves, we have no knowledge of any 

 such substance having the property of spontaneous multiplica- 

 tion. Nothing in fact explained the phenomena so well as 

 the idea that the contagium was a living organised substance. 

 Many accepted the theory, not as proved, but as a working 

 hypothesis which answered better than any other. 



It was in the year 1856 that Pasteur first published the 

 results of his researches into the nature of fermentation, into 

 the details of which I entered on the last occasion. You 

 will recollect that he showed that fermentation and de- 

 composition were invariably associated with the presence of 

 minute organisms, and that the insertion of the minutest 

 portion of liquid containing these organisms, immediately 

 started the fermentation process in another liquid, up to that 

 time quite pure and fresh. Pathologists saw at once the 

 resemblance of these processes to those as seen in disease. 

 The insertion of a small quantity of infecting liquid was 

 exactly similar in this respect to the inoculation of small pox 

 virus or vaccine. Chauveau of Lyons afterwards investi- 

 gated the nature of the liquid obtained from the small pox 



