THE LIQUEFACTION OF GASES 49 



repulsive ones, they help on the expansion, and 

 the energy so liberated appears as sensible heat, 

 the resultant rise of temperature depending on the 

 strength of the repulsion between the molecules. 



The porous plug experiment, to which we 

 have referred on the last page, was devised by 

 Professor William Thomson, afterwards Lord 

 Kelvin, and the late Dr Joule, for the purpose 

 of examining the amount and nature of these 

 inter-molecular forces, and of determining the 

 amount of deviation of various gases from the 

 ideal state, in which no such forces exist. If a 

 thermometer were filled with such a hypothetical 

 ideal gas, its indications would coincide exactly 

 with the absolute temperature scale, deduced by 

 Thomson from the principles of thermodynamics. 

 The knowledge of the deviation of any real gas 

 from the ideal state thus enables us to compare 

 the absolute scale with the scale of an actual 

 thermometer, using the expansion of the gas in 

 question as the thermometric property. The 

 great theoretical importance of the porous plug 

 experiment will now be manifest. 



Thomson and Joule found that air, and all 

 other gases except hydrogen, were cooled slightly 

 on passing the plug ; with hydrogen, on the other 

 hand, they obtained a still smaller heating effect. 

 Thus in hydrogen the molecules must on the 

 whole repel each other, while in air and similar 

 gases, the intermolecular forces must be attractive 

 ones. The amount of the effect was found to in- 

 crease in proportion to the difference of pressure 

 on the opposite sides of the plug. 



With air the cooling effect decreases as the 

 temperature is raised, and increases if the air 



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