528 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



supposed that steam above (say) 150 C. consists entirely 

 of hydrol, but that small quantities of dihydrol are formed 

 as the boiling-point is approached. Liquid water is obviously 

 a mixture consisting mainly of " water-molecules " ; but at 

 high temperatures " steam-molecules " are present in large 

 proportions, whilst " ice-molecules " become important as 

 the freezing-point is approached. Ordinary ice, like solid 

 nitrogen peroxide, is probably a homogeneous material, 

 consisting entirely of " ice-molecules," H 6 O 3 . But Tammann 

 by compressing to 2,500 atmospheres and cooling to 22C. 

 has frozen out a dense ice, which sinks instead of floating 

 in liquid air, but swells up and reverts to ordinary light ice 

 when warmed up to 130 C. ; this dense ice is probably 

 composed of the denser " water-molecules," and may be 

 regarded as pure dihydrol, H 4 O 2 . There are, however, four 

 or five varieties of dense ice and two or more of light ice 

 known (Tammann, Annual Reports, 1910, 1912, 1913; 

 Bridgman, Proc. Amer. Acad., 1912, 47, 441). 



B. THE CONDITIONS OF CHEMICAL CHANGE. 



H, B. Baker (1894) on the influence of moisture on the 

 dissociation of sal-ammoniac. In his earlier papers Deville 

 described dissociation as a " spontaneous decomposition " 

 by heat without the intervention of any chemical agency. 

 The later work of H. B. Baker and others has rendered it 

 doubtful whether dissociation can ever occur under such 

 conditions. In his experiments on the " Influence of 

 Moisture on Chemical Change "( Trans. Chem. Soc., 1894, 

 65, 611-624), H. B. Baker found that dry ammonium 

 chloride could be vaporised without decomposition into 

 the dried bulb of a Victor Meyer's vapour-density apparatus, 

 specially constructed of hard glass and heated to 350 C. 

 Six experiments gave an average vapour-density 27*8, 

 relatively to hydrogen, as compared with 2675 calculated 



