264 LIQUID HYDROGEN, 



the cause of the photooi-aphic action may be essentially physical. No 

 better illustration could l)c given of the rapid diminution of chemical 

 action at low temperatures than to remind you that fluorine gas, the 

 most active elementary body, under such conditions may be liquetied 

 and kept in glass vessels. 



The effect of a temperature of 21"^ absolute on the electric resistance 

 of the pure metals is a problem of great interest. In passing from the 

 melting point of ice to the boiling point of h>'drogen, pure platinum 

 loses resistance till only one-fortieth remains, and in the case of elec- 

 trolytic copper the remaining resistance is only one lifty-seventh of 

 what it was at starting. Such results suggest the approach to the con- 

 dition of what may be called relatively perfect electric conductivity 

 as. the zero of absolute temperature is approached. 



Liquid hydrogen is a nonconductor of electricity, and as regards 

 being an insulator for currents of high potential it is comparable to 

 that of liquid air. The properties of the liquid we have witnessed in 

 no way suggest the metallic character that chemists like Faraday, 

 Dumas, and Graham anticipated; and foi- the future hydrogen nmst be 

 classed with the nonmetallic elements. 



The liquefaction of hydrogen has been the consequence of some ten 

 years' devotion to low-temperature research. To many it may seem 

 that the results have been indeed costly in more ways than one. The 

 scientific worker who prepares the way for future development in this 

 sort of inquiry generally selects complicated methods, and is attracted 

 or diverted into man}- bypaths of investigation. He may leave to his 

 successors any credit that may be attached to cheapness and ease of 

 production of the agent of research — results that must invariably fol- 

 low. Liquid hydrogen is an agent of research which will enable us 

 to examine into the properties of matter at the lowest-maintained tem- 

 perature ever reached by man. Much work has still to be accomplished. 

 One of the most fascinating prf)])lems of the study of low temperatures 

 has been materially advanced. The interval separating us from the 

 zero of absolute temperature has been reduced to practically one-fourth 

 the value that it stood at when liquid air was the cooling agent. We 

 can produce in pure Helium instantaneous temperatures, bringing us 

 still nearer the goal. Now we can maintain a temperature within less 

 than 16"^ of this zero, and the investigator who will make the further 

 attempt to reduce this distance by an equivalent amount, thereby reach- 

 ing a steady temperature of 4:-' or 5'-' absolute, will indeed face a prob- 

 lem of almost insuperable difficulty. Well, let us take comfort in an 

 aphorism of Davy's: '""Fortunately for the active and progressive 

 nature of the human mind, even experimental research is only a method 

 of approximation to truth." 



The success of the demonstration has l)een largely due to the uni'e- 

 mitting exertions of my chief assistant, Mr. Robert Lennox, and to the 

 valuable aid given by Mr. J. W. Heath. 



