112 MOLECULAR MOTION AND ITS ENERGY 49 



The fact that the vapour-density is smaller at higher 

 temperatures and larger at lower temperatures is explained 

 in the same way ; for since the heat-motion loosens the bond 

 between the molecules the molecules will be lighter, and 

 therefore the gas or vapour will be specifically lighter at 

 the higher than at the lower temperatures. 



The behaviour of the thermal expansion-coefficients is 

 also directly explained. At lower temperatures not only do 

 the gaseous molecules separate more widely from each other 

 on the addition of heat, but they also split up and require 

 greater space for their greater number. At higher tempera- 

 tures, at which all the molecules have been already split up, 

 heating brings about merely an increase of speed as in ideal 

 gases ; all vapours and condensable gases must therefore at 

 high temperatures attain the same thermal coefficients of 

 expansion as the so-called permanent gases, while at lower 

 temperatures they expand more largely. 



If this explanation of the deviations is really true, a con- 

 clusion already drawn by Eegnault 1 from his observations 

 on the compressibility of gases must be unconditionally 

 considered as correct. If molecules that are bound together 

 are more and more separated by rise of the temperature, 

 there must be a temperature at which all move singly and no 

 further separation is possible ; at this temperature the ground 

 in question of the anomalies would fail, so that the only 

 cause of an anomaly that would remain is the circumstance 

 that the dimensions of the molecules in comparison with 

 their distances apart need not be vanishingly small, a cause 

 therefore which, as in hydrogen, would entail a deviation 

 in the opposite direction. According to this theory, there- 

 fore, as Begnault has already conjectured, every gas must 

 at a sufficiently high temperature exert a greater pressure 

 than would be expected by Boyle's law, and a less pres- 

 sure at lower temperatures, so that for every gas there will 

 be a certain temperature at which it strictly obeys this law. 

 Hydrogen would at very low temperatures behave just like 

 the others. 



1 Mini, de VAcad. de Paris, xxi. 1847, p. 404. 



