MOLECULES AND A.TOM8 313 



molecules of many of the elements prove to be complex for instance, 

 the weight of an atom of oxygeii=16, and its density=16, so that its 

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

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

 O ; j (Chap. IV.). So also, the molecule of hydrogen equals H 2 , of 

 chlorine C1 2 , of nitrogen N 2 , <tc. If chlorine react with hydrogen, the 

 volume remains unaltered after the formation of hydrochloric acid, 

 H 2 + Cl 2 =HCl-fHCl. 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 is 

 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 

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

 very complex molecule must be admitted, as otherwise its nonvola- 

 tility and other properties cannot be understood. And if compounds 

 are decomposed by a more of 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 NO 2 , 

 or ozone, O 3 , into ordinary oxygen, O 2 , then we might expect to 

 find the splitting- up of the complex molecules of elements into 

 the simplest molecule containing a single atom only that is to 

 say, if O 2 be obtained from O 3 , then the formation of O might also 

 be looked for. The likelihood of such a proposition is indicated 

 by the vapour of iodine. Its normal density =127 (Dumas, Deville, 

 arid others), which corresponds with the molecule I 2 . At tempera- 

 tures above 800 (up to which the density remains almost con- 

 stant), this density distinctly decreases, as is seen from the verified 

 results obtained by Victor Meyer, Crafts, and Troost. At the ordinary 

 pressure and 1000 it is about 100, at 1250 about 80, at 1400 about 

 75, and it apparently 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 1G of 66, as Crafts demon- 

 strated by reducing the pressure to 100 mm. and raising the temperature 

 to 1500. From this it may be concluded that at high temperatures and 

 low pressures the molecule I 2 gradually passes into molecule I containing 

 one atom like mercury, and that something similar occurs with other ele- 

 ments at a considerable rise of temperature, which tends to bring about 



16 Just as we saw (Chap. VI. Note 46) an increase of the dissociation of N 2 O 4 and the 

 formation of a large proportion of NO 2 , with a decrease of pressure. The splitting-up of 

 Io into I -f I is a similar dissociation. 



