LOW-TEMPERATURE RESEARCHES 



aggregate motion becomes less and less active, until 

 finally the molecule is moving so sluggishly that when 

 it collides with its fellow cohesion is able to hold it 

 there. Cohesion, then, has won the battle, and the gas 

 has become a liquid. 



Such, stated in terms of the mechanical theory of 

 heat, is what is brought to pass when a gas is liquefied 

 in the laboratory of the physicist. It remains only to 

 note that different chemical substances show the widest 

 diversity as to the exact point of temperature at which 

 this balance of the expansive and cohesive tendencies 

 is affected, but that the point, under uniform conditions 

 of pressure, is always the same for the same substance. 

 This diversity has to do pretty clearly with the size of 

 the individual molecules involved; but its exact ex- 

 planation is not yet forthcoming, and, except in a gen- 

 eral way, the physicist would not be able to predict 

 the "critical temperature" of any new gas presented 

 to him. But once this has been determined by ex- 

 periment, he always knows just what to expect of any 

 given substance. He knows, for example, that in a 

 mixture of gases hydrogen would still remain gaseous 

 after all the others had assumed the liquid state, and 

 most of them the solid state as well. 



These mechanical conceptions well in mind, it is clear 

 that what the would-be liquefier of gases has all along 

 sought to attain is merely the insulation of the portion 

 of matter with which he worked against the access of 

 heat-impulse from its environment. It is clear that 

 were any texture known which would permit a heat- 

 impulse to pass through it in one direction only, noth- 

 ing more would be necessary than to place a portion of 



