APPLICATION OF THE MOLECULAR HYPOTHESIS 91 



Each one of the enormous number of molecules in any given 

 sample of gas is moving, therefore, with an enormous velocity, 

 and the pressure exerted by gases (explained above as due to the 

 impact of the molecules on the walls of the containing vessel) 

 becomes more intelligible. In air, the number of molecules 

 striking a single square centimeter of surface per second would 

 fill no less than 12 liters. 



Liquefaction of Gases: Critical Temperature. All gases 

 can be liquefied by sufficient cooling and compression. This fact 

 compels us to suppose that, after all, even gaseous molecules 

 have some tendency to cohesion. This cohesion is, in general, 

 scarcely perceptible so long as the gas is warm and is diffuse but 

 it is possible, by careful measurement, to establish the fact that it 

 does cause slight deviations from Boyle's Law even under such 

 conditions. Thus, 2 liters of oxygen at 1 atmosphere pressure, 

 when subjected to 2 atmospheres pressure, give 0.9991 liters 

 instead of 1 liter. The additional contraction of 0.0009 liters 

 (0.9 c.c.) is to be regarded as due to the effect of cohesion when 

 the molecules are thus crowded closer together. The gases which 

 are more easily liquefied than is oxygen show greater 

 effects. Thus, 2 liters of sulphur dioxide at 760 mm., 

 when subjected to 2 atmospheres pressure, give only 0.974 

 liters, showing a contraction due to cohesion of 26 c.c. 

 These data refer to 0. At lower temperatures the con- 

 tractions due to cohesion become rapidly greater. 



We can readily understand, therefore, that when the 

 velocity of the molecules is sufficiently reduced by cool- 

 ing, and the molecules are brought sufficiently close 

 together, the tendency of the molecules to cohere causes YIG 38 

 the gas to condense and assume the liquid form. In 1869 

 Andrews found that carbon dioxide could be liquefied at by 

 38 atmospheres pressure, and at 30 by 71 atmospheres, but 

 that above 31.35 it could not be liquefied by any pressure. 



