NINETEENTH CENTURY. 17 



enough yeast could be produced to quickly start fermentation 

 in a dozen more; and so on, ad infinitum, in both cases. The 

 phenomena of the one following precisely the role of the phe- 

 nomena of the otfcpw* in every respect, only one was the fer- 

 mentation of sugar, and the other was the production of the 

 disease, Vaccina. 



Neither did the likeness stop here; but, as we shall see, it 

 went much further. It was then well known that a certain 

 amount of heat destroyed the power of yeast forever. Trial 

 showed that precisely the same thing happened with the 

 vaccine virus. Furthermore, it was well known that when 

 a solution of sugar has been under the process of fermentation 

 until that process has spontaneously ceased, a further fermenta- 

 tion could not be had by adding more yeast; although un- 

 fermented sugar may remain in the solution. The results 

 when completed are completed once for all; and for a new 

 trial a new mixture must be had. Or in other words, the 

 result of fermentation prevents fermentation of the same 

 character again taking place. These phenomena are again 

 repeated by the vaccine virus. A person once vaccinated is 

 rendered insusceptible to further vaccination; at least for 

 some years. All these facts taken together furnished the 

 strongest possible proof of the identity of the two processes, 

 at least in mode of operation; however wide the difference 

 in the results. And the well known fact, that in the great 

 majority of the then known contagious diseases, one attack 

 rendered the subject of it incapable of again taking the same 

 malady, served to extend the theory to all diseases of this class. 



NINETEENTH CENTURY. 



This is the condition in which we find the theory of zymotic 

 diseases at the beginning of the present century. To say that 

 this theory was universally accepted and satisfactory, would 



