698 PRRLIMINARY OBSERVATIONS. 



ever to differ in one respect from ammonium and the metals, 

 namely in combining readily with hydrogen, which element acts 

 towards them as a salt-radical. The supposed prevalence of such 

 radicals in the constitution of organic compounds has led M. 

 Liebig to define organic chemistry as the chemistry of com- 

 pound radicals ; the whole of which indeed as are of a basyle 

 character, including ammonium itself, may be properly assigned 

 to this department of the science. 



Many organic substances are highly complex and contain a 

 large number of atoms; a circumstance which renders them very 

 liable to change, and has led to the observation of peculiar 

 modes of decomposition among organic compounds, indicating 

 novel modes of the action of chemical affinity. 



COMPOSITION OF ORGANIC SUBSTANCES, AND METHOD OF 

 ANALYSIS. 



The elements which usually enter into organic substances are 

 few in number, namely carbon, hydrogen, oxygen and nitrogen. 

 Some organic substances contain only carbon and hydrogen, as 

 olefiant gas and other hydro-carbons ; more frequently carbon, 

 hydrogen and oxygen, as sugar, gum, many neutral bodies, and 

 most organic acids. To these a fourth element is added, 

 nitrogen, in the vegeto-alkalies and various other compounds 

 which belong more usually to the animal than vegetable divi- 

 sion ; indeed carbon prevails in the organic world, as silicon 

 does in the mineral, and as most minerals are silicates, so 

 organic substances are the compounds of carbon. To these 

 elements certain others are occasionally added, although most 

 usually by artificial processes, as chlorine in the place of 

 hydrogen, and sulphur, phosphorus, arsenic or tellurium in that 

 of oxygen. The elementary analysis of organic matters is deter- 

 mined with much exactness, and by means so simple and 

 rapid of execution as to render an ultimate analysis often the 

 most ready test of the purity of a substance. The process 

 followed, consists in burning the matter to be analysed by means 

 of oxide of copper, so as to convert its carbon into carbonic 

 acid and its hydrogen into water, which are both collected and 

 weighed ; when the matter contains nitrogen, the latter is col- 

 lected in the form of gas. The oxygen, which the matter contains, 



