320 PRINCIPLES OF CHEMISTRY 



molecule must contain two atoms, O 2 , which might already be Concluded? 

 by comparing its density with that of ozone, whose molecule contains^ 

 O 3 (Chapter IV.) So also the molecule of hydrogen equals H 2 , ofl 

 chlorine C1 2 , of nitrogen N 2 , &c. If chlorine react with hydrogen, the 

 volume remains unaltered after the formation of hydrochloric acid, 

 H 2 + C1 2 = HC1 + HC1. It is a case of substitution between the one, 

 and the other, and therefore the volumes remain constant. There are 

 elements whose molecules are much more complex for instance, sulphur,, 

 S 6 although, by heating, the density is reduced to a third, and S 2 isi 

 formed. Judging from the vapour density of phosphorus (D = 62}! 

 the molecule contains four atoms P 4 . Hence many elements when! 

 polymerised appear in molecules which are more complex than thei 

 simplest possible. In carbon, as we shall afterwards find, a very 

 complex molecule must be admitted, as^ otherwise its non-volatility and 

 other properties cannot be understood. And if compounds are de- 

 composed by a more or less powerful heat, and if polymeric substances 

 are depolymerised (that is, the weight of the molecule diminishes) by a 

 rise of temperature, as N 2 O 4 passes into N0 2 , or ozone, 3 , into ordinary 

 oxygen, O 2 , then we might expect to find the splitting-up of the com- 

 plex molecules of elements into the simplest molecule containing a 

 single atom only that is to say, if O 2 be obtained from 3 , then 

 the formation of O might also be looked for. The possibility but not 

 proof of such a proposition is indicated by the vapour of iodine. Its 

 normal density = 127 (Dumas, Deville, and others), which corresponds 

 with the molecule I 2 . At temperatures above 800 (up to which the 

 density remains almost constant), this density distinctly decreases, as 

 is seen from the verified results- obtained by Victor Meyer, Crafts, and 

 Troost. At the ordinary pressure and 1,000 it is about 100, at 

 1,250 about 80, at 1,400 about 75, and apparently it strives to reduce 

 itself to one-half that is, to 63. Under a reduced pressure this 

 splitting-up, or depolymerisation, of iodine vapour actually reaches 

 a density IG of 66, as Crafts demonstrated by reducing the 

 pressure to 100 mm. and raising the temperature to 1,500 From this 

 it may be concluded that at high temperatures and low pressures the 

 molecule I 2 gradually passes into the molecule I containing one atom 

 like mercury, and that something similar occurs with other elements at 

 a considerable rise of temperature, which tends to bring about the 

 disunion of compounds and the decomposition of complex molecules. 17 



16 Just as we saw (Chapter VI. Note 46) an increase of the dissociation of N 8 O 4 and the 

 formation of a large proportion of NO 2 , with a decrease of pressure. The decomposition 

 of Ij into I + 1 is a similar dissociation. 



17 Although at first there appeared to be a similar phenomenon in the case of 

 chlorine, it was afterwards proved that if there is a decrease ,of density it is only a small 



