90 CHEMICAL DISCOVERY AND INVENTION 



difficulties arise in their application. First of all it is necessary 

 to cool hydrogen gas to a temperature of about -80 C. before 

 the evolution of heat observed in the Joule-Thomson experi- 

 ments is changed into a cooling effect. Compression and release 

 of hydrogen gas, therefore, bring the experimenter no nearer to 

 the point of liquefaction unless the gas is already below that 

 temperature. Then the critical point for hydrogen is at 238 to 

 240 below zero, or considerably more than 100 below the 

 critical point of oxygen. There can be no liquefaction of 

 hydrogen, therefore, no matter what pressure is employed, 

 without the intense cooling obtained by causing liquid air to 

 boil rapidly under reduced pressure The self-intensive apparatus 

 can then be used successfully. T id hydrogen is a colourless 

 liquid with a well-defined surface, ; . having an extraordinarily 

 low density. Bulk for bulk it has only about T V the weight of 

 water. It boils at about -252 to -253 C., and by rapid evapora- 

 tion may be cooled till it sets into a wax-like mass resembling 

 solid paraffin. The appearance of this solid at once dissipates 

 the favourite theory advanced by Graham half a century or 

 more ago, that hydrogen gas is the vapour of an extremely 

 volatile metal. It certainly has many of the properties of a 

 metal in the production of salts (acids), and in the readiness 

 with which it exchanges places with metals in ordinary saline 

 reactions. The significance of this exchange is, however, dis- 

 counted by the fact that it also exchanges with chlorine and the 

 other halogens which are at the opposite end of the electro- 

 chemical scale. Hydrogen gas is more nearly the analogue of 

 marsh gas, CH 4 , and from one point of view may be regarded as 

 the first term of the series of hydrocarbons called paraffins. 

 Liquid hydrogen in quantity was first produced in an open 

 vessel by Sir James Dewar in the laboratory of the Royal 

 Institution in 1898. 



One gas now remained, namely, the inert gas helium, which, 

 discovered in 1895, was obtainable in moderate quantity. But 

 it was another ten years before this exception to the rule, which 

 could now be applied to gases generally, was abolished. Liquid 

 hydrogen was now available as a cooling agent, and the liquefier 

 being supplied with this powerful aid to condensation success 

 was at last achieved. 



To Professor H. Kameylingh-Onnes, in the cryogenic laboratory 

 directed for many years by him in the University of Leiden, 

 science ow^es this interesting result. More than 60 cubic centi- 



