METHODOLOGICAL INTRODUCTION. 13 



fluids and tissues, is similar to that of mineralogical chemistry, for 

 as in the one case, we seek for elucidation respecting the proximate 

 constituents of often highly complicated compound minerals and 

 rocks, so in the other we endeavour analytically to determine 

 the constitution of animal fluids and solid organised parts by the 

 aid of the knowledge we have already obtained from zoo-chemistry. 

 It was in these data that the nature of physiological and patho- 

 logical chemistry was formerly studied, and it was believed that the 

 processes themselves might be determined directly from the know- 

 ledge afforded by such analyses. The fallacy of such a view is proved 

 no less by the state of our knowledge, some ten years since, regard- 

 ing the physiology of nutrition and secretion, than by the numerous 

 errors propagated since that period in reference to the chemical 

 processes in the animal body. What were analyses of the blood, 

 urine, milk, and bile before this epoch, but mere isolated facts 

 deficient in those links that ought to bind them to the theory of 

 nutrition and secretion ? Physiology then regarded such analyses 

 more as mere accessories than as necessary means for the compre- 

 hension of each process. A more exact, although by no means a 

 perfect knowledge of the chemical qualities of these juices was sub- 

 sequently acquired, and hence it was attempted to establish a more 

 intimate relation between the chemical constitution and the phy- 

 siological function ; but from the absence of a proper analytical 

 foundation, this method not unfrequently led to numerous perver- 

 sions and dangerous errors, as we have already stated, and as we 

 might illustrate by a large number of examples. Although the 

 results of the chemical analysis of the animal juices may afford 

 many indications of the processes, they by no means enable us to 

 judge of the function itself, however numerous and complete they 

 may be ; and it is only by means of experiments founded on the 

 composition of these fluids that we are able to arrive at any satis- 

 factory conclusion regarding the nature of the processes in question. 

 The study of the zoo-chemical processes based on zoo-chemistry 

 and the theory of the animal juices, appertains to the third section 

 of physiological chemistry, the theory of the metamorphosis of 

 tissues of nutrition and secretion. It has already been observed, 

 that the actual object of physiological chemistry is to examine the 

 course of the chemical phenomena of the animal organism in their 

 causal connexion, and to deduce them from known physical and 

 chemical laws ; or in other words, to explain them scientifically. 

 Even if we regard the chemical substratum, as made known to us 

 by zoo-chemistry and the theory of the juices, in the light of a 



