The Romance of Chemistry 725 



about by the molecules of air, that it is sometimes going one way 

 and sometimes another, and like a hare, which is always doubling, 

 though it goes at a great pace, it makes very little progress." 

 Gradually, however, the ammonia gas does spread through the 

 air of the room. 



In a liquid, as contrasted with a gas, the molecule has hardly 

 any free path, but "is always in a state of close encounter with 

 other molecules." In a solid the molecules have almost lost their 

 opportunities for moving about. 



Now everyone knows that water-vapour may condense into 

 flowing water, and that this may freeze into solid ice, which may 

 melt again and steal off as mist. Saturated steam above 720.6 C. 

 is a gas. Thus water occurs in four states. The passage from 

 phase to phase is familiar, but in the liquefaction of gases it 

 became dramatic. 



About the beginning of the nineteenth century, Nopthmore 

 and others liquefied sulphurous acid gas by pressure, but progres- 

 sive research on the liquefaction of gases began with the work of 

 Faraday and Sir Humphry Davy, in 1823, when chlorine, 

 carbonic acid, ammonia, and other gases were liquefied by great 

 pressure. Later on Thilorier showed how the evaporation of a 

 jet of liquid carbonic acid produces cold so great that the rest of 

 the jet is frozen into fine snow. There is a temperature above 

 which no amount of pressure will produce liquefaction, and it was 

 not until devices for securing very low temperatures were dis- 

 covered and this was a great achievement of modern science 

 that it became possible to liquefy oxygen, nitrogen, and the like. 

 But with the invention of methods of obtaining very low tempera- 

 ture, which brings the molecules close together, just as heat drives 

 them apart, oxygen and nitrogen were conquered, and in 1898 

 Professor (now Sir James) Dewar produced liquid hydrogen. 

 As Professor Tilden remarks : 



It was both interesting and gratifying that the final 

 victory which crowned the long series of successful attacks 



VOL. Ill II 



