OKGANIC SUBSTANCES. 99 



lytic action by which starch is converted into sugar. The albumen 

 of the blood, by contact with the organic substance of the muscular 

 fibre, is transformed into a substance similar to it. The entire 

 process of nutrition, so far as the organic matters are concerned, 

 consists of such catalytic transformations. Many crystallizable 

 substances, which when pure remain unaltered in the air, become 

 changed if mingled with organic substances, even in small quantity. 

 Thus the casein of milk, after being exposed for a short time to a 

 warm atmosphere, becomes a catalytic body, and converts the sugar 

 of the milk into lactic acid. In this change there is no loss nor 

 addition of any chemical element, since lactic acid has precisely the 

 same ultimate composition with sugar of milk. It is simply a 

 transformation induced by the presence of the casein. Oily matters, 

 which are entirely unalterable when pure, readily become rancid at 

 warm temperatures, if mingled with an organic impurity. 



Fourthly, The organic substances, when beginning to undergo 

 decay, induce in certain other substances the phenomena of fer- 

 mentation. Thus, the mucus of the urinary bladder, after a short 

 exposure to the atmosphere, causes the urea of the urine to be con- 

 verted into carbonate of ammonia, with the development of gaseous 

 bubbles. The organic matters of grape juice, under similar circum- 

 stances, give rise to fermentation of the sugar, by which it is con- 

 verted into alcohol and carbonic acid. 



Fifthly, The organic substances are the only ones capable of 

 undergoing the process of putrefaction. This process is a compli- 

 cated one, and is characterized by a gradual liquefaction of the ani- 

 mal substance, by many mutual decompositions of the saline matters 

 which are associated with it, and by the development of peculiarly 

 fetid and unwholesome gases, among which are carbonic acid, 

 nitrogen, sulphuretted, phosphoretted, and carburetted hydrogen, 

 and ammoniacal vapors. Putrefaction takes place constantly after 

 death, if the organic tissue be exposed to a moist atmosphere at a 

 moderately warm temperature. It is much hastened by the presence 

 of other organic substances, in which decomposition has already 

 commenced. 



The organic substances are readily distinguished, by the above 

 general characters, from all other kinds of proximate principles. 

 They are quite numerous ; nearly every animal fluid and tissue 

 containing at least one which is peculiar to itself. They have not 

 as yet been all accurately described. The following list, however, 

 comprises the most important of them, and those with which we are 



