XXX PREFACE. 



from observation of the changes thus produced, 

 conclusions as to their behavior in the body. 



By pursuing this method, useful remedies or 

 modes of treatment might by accident be dis- 

 covered ; but a rational physiology cannot be 

 founded on mere reactions, and the living 

 body cannot be viewed as a chemical labo- 

 ratory. 



In certain diseased conditions, in which the 

 blood acquires a viscid consistence, this state 

 cannot be permanently removed by a chemical 

 action on the fluid circulating in the blood- 

 vessels. The deposit of a sediment from the 

 urine may, perhaps, be prevented by alkalies, 

 while their action has not the remotest tendency 

 to remove the cause of disease. Again, when we 

 observe, in typhus, insoluble salts of ammonia 

 in the faeces, and a change in the globules of 

 the blood similar to that which may be artifi- 

 cially produced by ammonia, we are not, on 

 that account, entitled to consider the presence 

 of ammonia in the body as the cause, but only 

 as the effect of a cause. 



Thus medicine, after the fashion of the Aris- 

 totelian philosophy, has formed certain concep- 

 tions in regard to nutrition and sanguification ; 

 articles of diet have been divided into nutri- 

 tious and non-nutritious ; but these theories, 

 being founded on observations destitute of the 



