300 ^00^' 



eould be refered only to the operation of this fluid, became desirous 

 ©f collecting it in purity, of analysing it, and of discovering, if 

 possible, by a chemical examination of its constituent principles, to 

 what its remarkable effects are owing. 



But, notwithstanding the labour and ingenuity that have been ex- 

 ercised upon the subject, by chemists and physiologists of the first 

 abilities and characters, both at home and abroad, very little (I should 

 be safe in saying almost nothing) has been done, towards developing 

 its constituent principles, much less towards explaining the mode in 

 which it acts in converting food into chyme. Many circumstances, 

 indeed, stand in the way of an accurate analysis of the gastric juice, 

 and prevent our coming to any definite or precise conclusions con- 

 cerning its nature and properties. 



In the first place, it has been found extremely difficult to collect 

 it in any considerable quantity, and still more so in a state of purity. 



It seems to me too_, that insuperable difficulties stand in the 

 way of obtaining it in a state of perfect purity, without which, all 

 attempts to explain its mode of action on food, must proceed upon 

 reasoning, purely hypothetical. 



Now, the very contradictory accounts, which have been given of 

 the nature and properties of the gastric juice, by those philosophers 

 who have attempted its chemical analysis, have arisen chiefly from 

 the impossibility of obtaining it totally unmixed with the food, and 

 with the other ecretions that are met with in the stomach. And if, 

 as has been suggested by some physiologists, there be good reason to 

 believe that this fluid is thrown into the stomach, only at the time 

 when food is present in the organ (a supposition which I think not 

 only exceedingly rational, but highly probable) then, all that has 



