148 NEW RESEARCHES ON LIQUID AIR. 



compounds led to the conclusion that although the atonic weight of 

 fluorine is nineteen times that of hydrogen, yet it must in the free state 

 approach hydrogen in volatility. This view'is confirmed by the atomic 

 refraction which Gladstone showed was 0.8 that of hydrogen, and from 

 which we may infer that the critical pressure of fluorine is relatively 

 small, like hydrogen. 1 If the chemical energy of fluorine at low tem- 

 peratures is abolished, like that of other active substances, then some 

 kind of glass or other transparent material could be employed in the 

 form of a tube, and its liquefaction achieved by the use of hydrogen as 

 a cooling agent. In any case a platinum vessel could be arranged to 

 test whether fluorine resists being liquefied at the temperature of solid 

 air, and this simple experiment, even if the result was negative, would 

 be of some importance. 



During the conduct of these investigations, I have gratefully to 

 acknowledge the able assistance rendered by Mr. liobert Lennox, my 

 chief assistant. Valuable help has also been given by Mr. J. W. Heath. 



iOn the other hand, the exceptionally small refractivity value observed by Lor^ 

 Rayleigh in the case of helium .shows that the critical pressure of this body is pro- 

 portionately high. It would therefore be more difficult to liquefy than a substance 

 having about the same critical temperature, but possessing a lower critical pressure, 

 like hydrogen. 



