Identification of Organic Compounds. 37 



The Molecular Weight of Compounds. 



In the earlier stages of the development of organic 

 chemistry it was considered an axiom that substances of 

 the same composition should possess the same chemical 

 properties. In 1825, however, Faraday discovered a 

 hydrocarbon in oil gas, which contained the same per- 

 centage of carbon and hydrogen as ethylene gas. It was 

 therefore obvious that the mere determination of the 

 relative proportions of the various elements present in a 

 compound was not sufficient for its complete characteriza- 

 tion. By means of analysis the relative but not the 

 absolute number of atoms present in a compound can be 

 ascertained. Thus, for example, a substance of the formula 

 C 2 H 4 would contain the same percentage of carbon and 

 hydrogen as a compound of the formula C 5 H 10 or C 6 H 12 . 

 It was not, however, until about 18401845 that the true 

 significance of molecular weight was recognized and an 

 older generalization of Avogadro was revived by Gerhardt 

 and Laurent and applied to organic compounds. Accord- 

 ing to Avogadro, all volumes of gases under the same 

 conditions of temperature and pressure contain the same 

 number of molecules ; the vapour of a substance of the 

 formula C 5 H 10 will, therefore, be heavier than that of a 

 substance C 2 H 4 . It is possible, then, directly to determine 

 the molecular weight of any substance which can be 

 obtained in a volatile form without decomposition, by 

 determining the density of its vapour. 



Determination of the Molecular Weight of Volatile 

 Substances. Various forms of apparatus can be employed 

 for this purpose, but the one most commonly used is that 

 suggested by Victor Meyer, which is figured in the ad- 

 joining sketch (Fig. 14a). A long narrow tube a, with 

 a reservoir at the bottom of the general form of a 



