88 VISCOSITY OF GASES 229 



creases when the temperature rises, and that of the com- 

 pound molecules when the temperature falls, until finally, 

 at a definite temperature, all are broken up and the per- 

 fectly gaseous state is attained. At lower temperatures, on 

 the contrary, the number of the compound molecules is in 

 excess. 



This behaviour of the molecules explains, as before re- 

 marked, the fact that the density of vapours, when compared 

 with the density of air or of any other gas under the same 

 pressure but at different temperatures, does not remain con- 

 stant. Since, in accordance with Avogadro's law, the 

 number of molecules in unit volume is the same for all 

 gases at the same pressure and temperature, we find a 

 smaller value of the vapour-density if the molecules are 

 broken up into smaller ones at higher temperatures ; at 

 lower temperatures, on the contrary, when all the molecules 

 are joined together, either in pairs or larger aggregates, the 

 vapour-density will turn out much larger. 



A change of pressure has just the same kind of effect 

 upon the degree of dissociation attained. If the pressure 

 is diminished by an enlargement of the volume, the mole- 

 cules separate from each other in greater number because 

 they acquire greater freedom of movement, and are less 

 exposed to the forces of cohesion. But an increment of 

 pressure brings the particles nearer together, and affords 

 them more frequent opportunity of combining. 



If these views are right, the law of alteration of the 

 density of a vapour with its pressure and temperature must 

 be determinable from theoretical considerations that go more 

 deeply into the nature of the phenomena. The first who 

 succeeded in finding this law was J. W. Gibbs. 1 In 

 addition to the general laws of gases he employed the two 

 principal laws of Thermodynamics, viz. the theorem of 

 energy and the theorem of entropy ; from these he deduced 

 formulae that agree excellently with experiment. Van der 



1 Trans. Connecticut Acad. New Haven 1876, iii. pt. 1, p. 234. Under the 

 title of ' Thermodynamical Studies, 1892,' published in German by Ostwald, 

 p. 204. Report on this memoir by Maxwell in Proc. Camb. Phil. Soc. ii. ; 

 Amer. Journ. Sci. [3] xiii. p. 380 ; Scientific Papers, ii. p. 498. 



