130 Drs. Kamsay and Young on 



panied by greater internal vibration, which eventually leads, 

 in almost all cases, to a simplification or rearrangement of the 

 molecules, involving chemical change. When increased 

 molecular motion is imparted to gases at temperatures much 

 above their points of condensation, and at moderate pressures, 

 the problem is a comparatively simple one ; and has been 

 solved with great success by Clausius, Maxwell, Thomson, 

 and others, from the physical side, and from the chemical side 

 by Pfaundler, Naurnann, and Willard Gibbs. But near their 

 condensing points, and also at high pressures, Boyle's and 

 Gay-Lussac's laws no longer hold, owing partly no doubt to 

 the mutual attraction of the molecules, and also to the fact 

 that the absolute size of the molecules is no longer insigni- 

 ficant relatively to the space which they occupy. Both these 

 causes of deviation may be relegated to the class " physical," 

 inasmuch as the mutual attraction alluded to is not confined 

 to any small number of molecules, but is exercised by each 

 molecule on all its neighbours, and limited in absolute amount 

 only by the relative masses of the attracting molecules 

 and by their distances from each other. But it is also con- 

 ceivable that this attraction may be wholly or in part of a 

 chemical nature, tending towards the formation of complex 

 molecules, resulting from combination of two or more simple 

 molecules. Now as this deviation from the simple gaseous 

 laws occurs both with what are commonly termed " stable " 

 and with " dissociable " substances, it is of importance to 

 enquire whether the abnormality of the vapour- density of 

 stable substances is at all due to chemical association of mole- 

 cules ; and how much of the abnormality of dissociable sub- 

 stances is to be ascribed to purely physical attraction of the 

 molecules for each other, due to mere propinquity. 



At any temperature below the critical one, when the volume 

 of gas is decreased, pressure rises until a certain maximum is 

 attained, when it becomes constant, and change of state 

 occurs. It is conceivable, on the one hand, that the liquid 

 condition is a purely physical one, and that a liquid consists 

 of molecules similar in all respects to those of its gas, but, 

 owing to their closer proximity, exhibiting that form of at- 

 traction which is known as cohesion. And on the other hand, 

 it has been advanced by Naumann and others that the gaseous 

 molecules, in changing to liquid, form molecular groups of 

 definite complexity, exercising cohesive attraction on each 

 other ; and, according to this view, the problem is both a 

 physical and a chemical one. According to the first view, if 

 heat be imparted to a liquid, work is done in expansion 

 against pressure, and in overcoming cohesion ; and, according 



