THE PRINCIPLES OP ORGANIC CHEMISTRY 41 



possess in common, they all contain carbon, and perhaps the 

 best definition of organic chemistry is, the chemistry of the 

 carbon compounds. It is remarkable that the compounds of 

 carbon by far exceed in number the compounds of all the other 

 elements, and the reason for this is to be sought in the nature 

 of the carbon atom itself. In order to understand this we must 

 consider a further general property of atoms, viz., what is 

 known as their valency, and for this purpose we must clearly 

 understand the meaning of, and the method of determining, 

 a molecular formula. 



We have already seen how, by determining the weights 

 of equal volumes of substances in the gaseous state, as 

 compared with the weight of an equal volume of hydrogen, 

 it is possible to determine the weight of a molecule of the 

 substance. By suitable methods of analysis we can de- 

 termine also the proportion by weight of any element in 

 that compound and thus obtain its molecular formula, 

 just as we have found that the molecular formula for 

 steam is HgO. Again, by burning a Imown weight of car- 

 bon in oxygen, determining the weight of carbon dioxide 

 produced, and by knowing also the weight of a volume 

 of this gas as compared with the weight of an equal 

 volume of oxygen, we find that 12 parts of carbon 

 unite with 32 parts by weight of oxygen to form a gas 

 the molecular weight of which is 44, and consequently its 

 molecular formula is COj. Knowing thus the molecular 

 weight of steam and of carbon dioxide and their molecular 

 formulae, viz., HjO and COg, we are in a position to determine 

 the molecular formulae of many organic compounds. 



On burning a given weight of a substance containing carbon 

 and hydrogen, the carbon is burnt to COg, and the hydrogen 

 to HjO, which may be respectively weighed ; from the 

 weights of CO2 and HgO formed, we can calculate the weight 

 of carbon found in the original compound taken, and thus 

 obtain its percentage composition. This method of analysis 



