22 ANIMAL CHEMISTRY LECTURE II. 



(19.) CHEMISTS have ascertained that the various tissues of plants 

 and animals are composed of, or contain, a great number of dis- 

 tinct chemical compounds, capable, for the most part, of being sepa- 

 rated from one another by what may be regarded as physical pro- 

 cesses that is to say, by processes dependent on differences of 

 volatility, fusibility, solubility in different menstrua, &c. These 

 several compounds have either been built up in the living plant 

 or animal, or been formed spontaneously in the dead plant or 

 animal out of ancestral substances which were themselves built 

 up in the living plant or animal. Somehow or other these proxi- 

 mate animal and vegetable principles, as they are termed, have 

 been produced through the agency of vitality. They have been 

 formed through the intervention of living organisms, and are 

 hence called organic compounds, in contradistinction to such 

 substances as quartz and feldspar and haematite, which pre-exist 

 in the mineral kingdom, and from such substances as copperas 

 and alum and carbonate of soda, which are produced artificially 

 by human ingenuity out of the pre-existing compounds of the 

 mineral kingdom. 



(20.) When the chemist gets hold of these different tissue pro- 

 ducts and components he submits them to a variety of experi- 

 ments, and subjects them to the most strange transformations ; he 

 performs a simple subtraction, by taking away certain constituent 

 atoms and leaving the remainder ; or he performs a simple addi- 

 tion, by affixing other constituent atoms, whether of the same 

 or a different nature ; or he performs a substitution, taking away 

 certain constituent atoms and introducing fresh ones in their 

 places ; or he effects a more or less complete decomposition, by 

 breaking up the original substance into a variety of less complex 

 bodies. Now, all these products into which the chemist trans- 

 forms the proximate vegetable and animal principles, of which 

 we have spoken, belong to the class of organic compounds. As 

 a rule, they do not pre-exist in living organisms, they are not 

 formed spontaneously in dead organisms, but they result from 

 the skill of the chemist operating upon compounds which were 



