344 DIRECT PROPERTIES OF MOLECULES 123 



In order, therefore, to explain the behaviour of the 

 gaseous particles, we must have recourse to the assumption 

 of forces which the particles exert on each other, at least 

 when near enough ; and there arises only the question 

 whether these forces are attractive or repulsive ; for the 

 latter may also come into play when the distance becomes 

 small enough, though the gaseous molecules certainly attract 

 each other at their mean distances of separation. 



On this account Maxwell, too, for a long time de- 

 fended the assumption of repulsive forces between gaseous 

 particles. These repulsive forces were supposed to decrease 

 more rapidly than the attractive forces as the distance in- 

 creased, and, indeed, inversely in proportion to the fifth power 

 of the distance, while the attractive forces were to be taken 

 as following Newton's law of being inversely proportional 

 to/the square of the distance. Maxwell 1 arrived at this 

 hypothesis, which allows of a very easy and elegant treat- 

 raent, because the law of dependence of the viscosity of a 

 gas on the temperature, which is deduced from this hypo- 

 thesis, is that which he obtained by experiment. But since 

 this law, viz. that the coefficient of viscosity of a gas is pro- 

 portional to the absolute temperature, is contradicted by 

 later observations, the hypothesis must be given up. 



Other physicists have professed the view that attractive 

 forces are to be assumed as acting between gaseous mole- 

 cules. That such forces must be assumed unconditionally 

 has been proved by Clausius by means of his proposition 

 of the Virial, from which it follows that a stationary state 

 of motion can be permanently maintained only if forces 

 maintain the equilibrium dynamically, and that therefore a 

 stable state is quite impossible without attractive forces. 2 

 Further, Boltzmann 3 has shown that it is also sufficient 

 to assume only attractive forces, and no repulsive forces, 

 between gaseous molecules. By assuming that strongly 



1 Phil. Trans. 1867, clvii. p. 49 ; Phil. Mag. 1868 [4] xxxv. p. 129 ; Sclent. 

 Papers, ii. p. 26. 



- Bull. Ac. Belg. 1886 [3] xi. p. 193 ; Kinetische Theor. d. Gasc, 1889-91, 

 p. 264. 



3 Wien. Sitzungsber. 1884, Ixxxix. Abth. 2, p. 714 ; Wied. Ann. 1885, xxiv. 

 p. 37. 



