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another into mannite and gum. If the putrid decomposition 

 of such simple mixtures as sugar and yeast he so far altered 

 by a few degrees of temperature as to produce compounds so 

 widely different from each other as alcohol and lactic acid, 

 how much more likely will it be to exercise a similar influence 

 on the heterogeneous mixtures which constitute our undrained 

 marshes or stagnant receptacles of filth ? If we have poison- 

 ous matters produced from the decomposition of sugar at one 

 temperature, and salutary ones at another, is it not reasonable 

 to infer that such would be the case during the continuous 

 and ever-varying changes which such matters are constantly 

 undergoing? I do not think it improbable that the mode 

 of decomposition in such matters may be altered by every 

 change of temperature ; hence we may have the same places 

 at one time producing compounds comparatively harmless, and 

 at another engendering the worst form of infectious disease. 



That various kinds of epidemic disease are produced by 

 contact with matters of a similar composition, although in a 

 different state of change, is evident from the small pox virus, 

 the matter of plague, &c. The remarkable manner in which 

 such contagious matters reproduce themselves in the diseased 

 subject would lead us to infer that if epidemic fever arose 

 from the action of putrid malaria, — if it arose from a state 

 of decomposition, communicated to the blood by contact^ the 

 modus operandi of its action would be precisely similar to 

 that of contagious disease. We should have poisonous 

 matters re-generated in the system, which would only differ 

 from those of contagion by being volatile, and which would 

 consequently infect the atmosphere by exudation from the 

 lungs and skin. The reproduction of the exciting agent is 

 well illustrated in the action of yeast in a glutinous solution 

 of sugar, such as wort ; and in this we have a neutral sub- 

 stance in solution, which, during the fermentation, is itself 

 converted into yeast. I may say that the gluten here becomes 



