454 ATOM. 



perimente of Rtgnault and others, that the pressure does deviate from Boyle's 

 kw when the density of the gas is increased. In the case of carbonic acid 

 and other gases which are easily liquefied, this deviation is very great. In ail 

 case*, howerer, except that of hydrogen, the pressure is less than that given 

 by Boyle's law, shewing that the virial is on the whole due to attractive forces 

 between the molecules. 



Another kind of evidence as to the nature of the action between the 

 molecules is furnished by an experiment made by Dr Joule. Of two vessels, 

 one was exhausted and the other filled with a gas at a pressure of 20 atmo- 

 spheres; and both were placed side by side in a vessel of water, which was 

 constantly stirred. The temperature of the whole was observed. Then a com- 

 munication was opened between the vessels, the compressed gas expanded to 

 twice its volume, and the work of expansion, which at first produced a strong 

 current in the gas, was soon converted into heat by the internal friction of 

 the gas. When all was again at rest, and the temperature uniform, the 

 temperature was again observed. In Dr Joule's original experiments the ob- 

 served temperature was the same as before. In a series of experiments, con- 

 ducted by Dr Joule and Sir W. Thomson on a different plan, by which the 

 thermal effect of free expansion can be more accurately measured, a slight 

 cooling effect was observed in all the gases examined except hydrogen. Since 

 the temperature depends on the velocity of agitation of the molecules, it 

 appears that when a gas expands without doing external work the velocity of 

 agitation is not much affected, but that in most cases it is slightly diminished. 

 Now, if the molecules during their mutual separation act on each other, their 

 velocity will increase or diminish according as the force is repulsive or attractive. 

 It appears, therefore, from the experiments on the free expansion of gases, 

 that the force between the molecules is small but, on the whole, attractive. 



Having thus justified the hypothesis that a gas consists of molecules in 

 motion, which act on each other only when they come very close together 

 during an encounter, but which, during the intervals between their encounters 

 which constitute the greater part of their existence, are describing free paths, 

 and are not acted on by any molecular force, we proceed to investigate the 

 motion of such a system. 



The mathematical investigation of the properties of such a system of 

 molecules in motion is the foundation of molecular science. Clausius was the 

 first to express the relation between the density of the gas, the length of 



