493 



which would be generated without it. The presence of 

 many vegetable acids in the fermenting juice of the grape 

 gives rise to volatile compounds, which produce the flavour 

 and bouquet of wine, no such compound being produced 

 by the simple fermentation of sugar (the addition of rennet 

 to milk rapidly converts its sugar into lactic acid) ; hence we 

 may safely infer that the various mixtures which are con- 

 stantly occurring in our stagnant house-drains and cesspools, 

 as well as the extent to which these matters are diluted with 

 water, will, at one time, so far modify or alter their mode of 

 transformation, as to produce compounds which are highly 

 injurious to health, while at another they would be quite 

 harmless. It is not in all states of putrefaction that matter 

 introduced from dissection cuts is capable of producing 

 dangerous symptoms; it is only when the putrefaction assumes 

 a particular form that such is the case. The same thing 

 may certainly occur with putrid drains, &c. We shall at all 

 times find putrid matters present in the air contiguous to 

 such drains, but the decomposition will not always be of such 

 a character as to produce disease. 



Perhaps the principal agent which modifies the process of 

 putrefaction, and determines the manner in which the elements 

 of compounds shall arrange themselves to form new products, 

 is temperature. Nothing is more common in chemical opera- 

 tions than to produce one compound from a certain mixture 

 at a low temperature, and another at a high one. In the case 

 of yeast acting in a solution of sugar producing carbonic acid 

 and alcohol, it is only below the temperature of 80** that this 

 decomposition occurs. At a temperature between 80° and 100% 

 the transformation of the yeast and sugar is so far changed, 

 that instead of carbonic acid and alcohol, we have lactic 

 acid and gum. In the same manner we have the elements of 

 sugar of milk at one temperature dividing themselves into 

 alcohol and carbonic acid, at another into lactic acid, and at 



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