312 PRINCIPLES OF CHEMISTRY 



deviations appear in reality as an excellent proof of the general applica- 

 tion and significance of the law of Avogadro-Gerhardt. 



In those cases where the quotient vy proves to be less than 2, or 



the observed density greater than that calculated by a multiple number 

 of times, the matter is evidently more simple, and the fact observed 

 only indicates that the weight of the molecule is as many times greater, 

 as that taken as the quotient obtained is less than 2. So, for instance, 

 in the case of ethylene, whose composition is expressed by CH 2 , the 

 density was found by experiment? to be 14, and in the case of amy- 

 lene, whose composition is also CH 2 , the density proved to be 35, and 

 consequently the quotient for ethylene=l, and for amylene=?. If 

 the molecular weight of ethylene be taken, not as 14, as might be 

 imagined from its composition, but as twice as great namely, as 28 

 and for amylene as five times greater that is as 70 then the molecular 

 composition of the first will be C 2 H 4 , and of the second C A H 10 , and for 



both of them ^j- will be equal to 2. This application of the law, 



which at *first sight may appear perfectly arbitrary, is nevertheless 

 strictly correct, because the amount of ethylene which reacts for 

 example, with sulphuric and other acids is not equal to 14, but to 28 

 parts by weight. Thus with H 2 SO 4 , Br 2 , or HI, &c., ethylene com- 

 bines in a quantity C 2 H 4 , and amylene in a quantity C ft H 10 , and not 

 CH 2 . On the other hand, ethylene is a gas which liquefies with diffi- 

 culty (absolute boiling point= + 10), whilst amylene is a liquid boiling 

 at 35 (absolute boiling point= + 192), and by admitting the greater 

 density of the molecules of amylene (M=70) its difference from the 

 lighter molecules of ethylene (M = 28) becomes clear. Thus, the 



smaller quotient ^y is an indication of polymerisation, as the larger 



quotient is of decomposition. The difference between the densities of 

 oxygen and ozone (Chap. IV.) is a case in point. 



On turning to the elements, it is found in certain cases, especially 

 with metals for instance, mercury, zinc, and cadmium that that 

 weight of the atoms which must be acknowledged in their compounds 

 (of which mention will be afterwards made) appears to be also the 

 molecular weight. Thus, the atomic weight of mercury must be taken 

 as=200, but the vapour density=100, and the quotient=2. Con- 

 sequently, the molecule of mercury contains one atom, Hg. It is the same 

 with sodium, cadmium, and zinc. This is the simplest possible molecule, 

 which necessarily is only possible in the case of elements, as the mole- 

 cule of a compound must contain at least two atoms. However, the 



