A HISTORY OF SCIENCE 



average distance, and we term the condition the liquid 

 state. But if the heat impulse preponderates, the 

 molecules (unless restrained from without) fly farther 

 and farther asunder, moving so actively that when 

 they collide the recoil is too great to be checked by 

 cohesion, and this condition we term the gaseous 

 state. 



Now after this statement, it is clear that what the 

 low-temperature worker does when he would liquefy 

 a gas is to become the champion of the force of co- 

 hesion. He cannot directly aid it, for so far as is 

 known it is an unalterable quantity, like gravitation. 

 But he can accomplish the same thing indirectly by 

 weakening the power of the rival force. Thus, if he 

 encloses a portion of gas in a cylinder and drives a pis- 

 ton down against it, he is virtually aiding cohesion by 

 forcing the molecules closer together, so that the hold 

 of cohesion, acting through a less distance, is stronger. 

 What he accomplishes here is not all gain, however, 

 for the bounding molecules, thus jammed together, 

 come in collision with one another more and more 

 frequently, and thus their average activity uf vibration 

 is increased and not diminished; in other words, the 

 temperature of the gas has risen in virtue of the com- 

 pression. Compression alone, then, will not avail to 

 enable cohesion to win the battle. 



But the physicist has another resource. He may 

 place the cylinder of gas in a cold medium, so that the 

 heat vibrations sent into it will be less vigorous than 

 those it sends out. That is a blow the molecule cannot 

 withstand. It is quite impotent to cease sending out 

 the impulses however little comes in return ; hence the 



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