xx DISSOCIATION 537 



Above 1 50 C. nitrogen peroxide consists entirely of the brown 

 gas NO 2 ; at 100 C. there is I of N 2 O 4 by weight, at 50 C. f, and 

 at 27 C. f. The liquid (b.p. 22 C.) consists almost entirely of 

 N 2 O 4 , which finally freezes out at - 10 C. as a colourless ice. 

 The dissociation of the colourless tetroxide is accompanied by an 

 absorption of heat, and produces a remarkable increase in the 

 heat-capacity of the gas. 



Similar phenomena occur in the case of water. The vapour 

 at 100 C. consists of " steam-molecules," H 2 O, mixed with about 

 9% of " water-molecules," H 4 O 2 . The liquid is probably a 

 similar mixture, but consisting mainly of " water-molecules," 

 H 4 O 2 , in which at lower temperatures " ice-molecules" (perhaps 

 H 6 O 3 ) of lower density are formed in increasing proportions, 

 giving rise finally to an expansion of volume between 4 and o C. 

 Ordinary light ice is evidently composed of the same "ice- 

 molecules" in the crystalline state, but the dense ice, which 

 freezes out under pressures above 2,300 atmospheres (Tammann ), 

 may be composed of the denser " water-molecules." There are 

 probably 4 or 5 varieties of dense ice, one of which melts at 

 78 C C. under 20,000 atmospheres pressure (Bridgman) : it is 

 also possible that light ice exists in several forms (Tammann). 



B. THE CONDITIONS OF CHEMICAL CHANGE 



Wanklyn, in 1869, showed that dry chlorine did not act on 

 solid or on molten sodium ; this observation was confirmed, and 

 extended to other metals, by Cowper in 1883. 



H. B. Dixon, in 1884, found that a dried mixture of carbonic 

 oxide and oxygen was not explosive. 



H. B. Baker, in 1888, showed that sulphur and phosphorus 

 could be distilled and that purified carbon could be heated to 

 redness in dry oxygen without ignition. In 1894 he showed that 

 dry ammonium chloride could be vaporised without dissociating 

 and that it sublimed unchanged from quicklime ; conversely, dry 

 ammonia and dry hydrogen chloride did not combine when 

 mixed. Sulphur trioxide did not combine with quick- 

 lime or with cupric oxide. Dry nitric oxide did not combine 

 with oxygen. In 1900, he showed that dry calomel could be 

 vaporised, as Hg 2 Cl 2 , without decomposition and that the vapour 

 did not amalgamate gold-leaf : the later work of A. Smith has 



