XIV PREFACE. 



By pursuing this method, useful remedies or 

 modes of treatment might by accident be disco- 

 vered ; but a rational physiology cannot be founded 

 on mere re-actions, and the living body cannot be 

 viewed as a chemical laboratory. 



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, inso- 

 luble salts of ammonia in the faeces, and a change 

 in the globules of the blood similar to that which 

 may be artificially produced by ammonia, we are 

 not, on that account, entitled to consider the pre- 

 sence of ammonia in the body as the cause, but 

 only as the effect of a cause. 



Thus medicine, after the fashion of the Aristote- 

 lian philosophy, has formed certain conceptions in 

 regard to nutrition and sanguification ; articles of 

 diet have been divided into nutritious and non- 

 nutritious ; but these theories, being founded on 

 observations destitute of the conditions most essen- 

 tial to the drawing of just conclusions, could not be 

 received as expressions of the truth. 



