THE LIQUEFACTION OF GASES 47 



machines. The essential feature in the process 

 of cooling by the performance of external work is 

 the expansion of the gas by its own elastic force. 

 If the work necessary for the increase of volume 

 under the external pressure be supplied by an 

 engine, or if all such work be prevented by making 

 the gas expand into a vacuum, there is no external 

 work to absorb the heat energy of the gas itself, 

 and no cooling from this cause is produced. The 

 gas, however, still has to supply any work needed 

 to separate its own particles against any mutual 

 attractive forces, and, if such forces exist, cooling 

 can still be obtained at the expense of the heat- 

 energy of the gas. On the other hand, if the inter- 

 molecular forces are forces of repulsion, expansion 

 will be aided by their action, and will, in the 

 absence of external work, be accompanied by an 

 increase of temperature. Thus, by arranging for 

 free expansion, as It is called, we can examine the 

 nature of the inter-molecular forces by observing 

 whether a gas is cooled or heated. 



In such experiments, it is necessary to prevent 

 the performance of external work by the gas itself, 

 and this can be done in either of the two ways 

 indicated above. Gay Lussac, and afterwards 

 Joule, filled one vessel with gas under high 

 pressure, and then allowed the gas to expand 

 into another vessel previously exhausted. Here, 

 in expanding into a vacuum, no external pressure 

 has to be overcome, and no external work is done. 

 Any thermal change will be the equivalent of the 

 internal work. The vessels were placed side by 

 side in water, which was stirred after the experi- 

 ment, and tested with a sensitive thermometer. 

 At ordinary temperatures no heating or cool- 



