METHODOLOGICAL INTRODUCTION. 11 



and mode of decomposition, its preparation, the method of testing 

 for it, and its quantitative determination ; in explaining the physio- 

 logical relations of each substance, we shall endeavour to deter- 

 mine its occurrence in the animal body, and its origin, (whether it 

 be produced within or without the body,) and from the above con- 

 siderations, we shall finally attempt to deduce its physiological value. 

 We shall treat of the properties of each organic substratum 

 before considering the remaining chemical relations, as it appears 

 to us both unpractical and illogical to begin with the mode of pre- 

 paration, as is usually done ; unpractical, because no student can 

 comprehend the mode of preparation when he is not in some degree 

 acquainted with the properties of the substance in question, and 

 illogical, because we must have some idea of a body before we can 

 attempt to prepare or exhibit it. The composition of a body must 

 necessarily constitute the most important subject of consideration 

 after its properties and its principal reactions have been duly noticed, 

 for it is only by such means that we can attain to an idea of the 

 nature of a substance, and of the place it occupies in the system 

 of organic chemistry. Hence this section must not be limited to 

 a mere enumeration of analyses or of empirical formulae, but must 

 embrace a consideration of the arguments that are adducible in 

 favour of the different views of the theoretical internal constitution 

 of a substance, and which are briefly expressed by the rational 

 formula. This method is of the greatest importance for the 

 recognition of the physiological relations of organic substances; 

 since without it, we are unable to arrive at any logically cor- 

 rect judgment regarding the origin and the physiological importance 

 of different substances. If a knowledge of the composition of 

 an organic substance were not necessary to the investigation of 

 its combinations and products of decomposition, we should have 

 placed it after the latter, since they constitute the safest grounds 

 from which we may form an opinion of the rational composition 

 of a body. A careful study of the products of decomposition 

 is however the more necessary, since it is mainly on these that 

 we must base our view of the metamorphoses experienced by any 

 given substance within the vital sphere. 



It is only when all these relations have been considered, that 

 we shall deem it expedient to enter upon the different methods of 

 preparation or exhibition, for then only can the directions given 

 for the separation of substances be understood. 



Before considering a substance from a physiological point of 

 view, we must examine the means by which we are best able to 



