ORGANIC OR CARBON COMPOUNDS. 2J 



formed at some time or other through the agency of living organ- 

 isms. Just as alum and carbonate of soda, which the chemist 

 manufactures out of native minerals, belong to the class of mineral 

 compounds, so do such substances as chloroform and aniline and 

 cyanuric acid, which the chemist manufactures out of the proxi- 

 mate principles of plants and animals, belong to the class of 

 organic compounds. 



(21.) It is found that all organic compounds, whether of na- 

 tural or artificial production, contain carbon as an essential 

 constituent ; nearly all of them contain hydrogen also ; while the 

 great majority consist of carbon, hydrogen, and oxygen. In my 

 last lecture I brought under your notice certain nitrogenous pro- 

 ducts of tissue metamorphosis, but confining our present attention 

 to such organic bodies as consist of carbon, hydrogen, and oxygen, 

 or of carbon and hydrogen only, I would speak to you, in the 

 first place, of their immense number and variety. If we take 

 any three elements whatsoever, exclusive of carbon, we shall find 

 that by their mutual combination they very rarely indeed form 

 more than half-a-dozen definite and distinct compounds ; but we 

 are acquainted with some thousands of compounds composed 

 solely of carbon, hydrogen, and oxygen united with one another 

 in different quantities and proportions ; which thousands of com- 

 pounds differ most strikingly in their properties, but were all 

 produced originally in living organisms, or made artificially by a 

 transformation of antecedent compounds produced originally in 

 living organisms. 



(22.) In addition to their number and variety, organic or 

 carbon compounds are characterised by the complexity of their 

 constitution ; that is, by the number of constituent atoms of 

 which their respective molecules are composed. If we take, any 

 definite mineral substance containing only three different kinds of 

 elementary matter, corresponding to the carbon, hydrogen, and 

 oxygen of the bodies now under consideration, we shall find that 

 the number of constituent atoms in such mineral substance very 

 rarely indeed exceeds ten or twelve, never perhaps reaches 



