POPULAR SCIENCE 481 



which will exert their influence across the unit plane. Intro- 

 ducing the above expression for ir into the modified gas equa- 

 tion we get an expression known as van der Waal's equation, 

 viz. (P + a/V 2 )(V-b)=RT. 



It would be quite beyond the scope of this article to deal 

 with the ramifications of this expression, and the very full 

 experimental tests to which it has been subjected. Suffice 

 it to say that, in many respects, it gives a wonderfully good 

 representation of the general behaviour of gases and liquids. 

 Notably it has served to indicate the conditions under which 

 transition from the liquid to the gaseous state can take place. 

 It affords a theoretical basis for the principle of continuity of 

 state, first enunciated on experimental grounds by Andrews, 

 according to which there is for every substance a certain 

 critical temperature above which it is impossible to condense 

 the gas to the liquid form, howsoever great the degree of com- 

 pression may be. At the same time the equation breaks 

 down in certain important quantitative aspects. For this 

 reason, other expressions have been proposed to account for 

 the temperature-pressure-volume relations of gases and liquids 

 of which the most noteworthy is that of Dieterici (5). It 

 cannot be said, however, that we have even yet attained to a 

 really satisfactory representation of the behaviour of such 

 systems under various conditions of temperature and pressure. 



Before leaving the van der Waals equation it is important 

 for our present purpose to point out that the assumption, 

 that the internal pressure or cohesive force per unit area inside 

 a liquid or compressed gas can be expressed as inversely pro- 

 portional to the square of the volume, really amounts to a 

 law of force for the attraction between any pair of molecules, 

 which varies as the inverse fourth power of the distance from 

 the centre of the molecule. This point has been very fully 

 examined by Sutherland, more especially in view of the fact 

 that an inverse fourth power law is the one to be expected 

 for an electrical doublet, consisting of a positive and a nega- 

 tive charge, acting upon a similar doublet. The evidence 

 brought forward in this connection is fairly convincing, and 

 leads to the conclusion that molecular attractions are essen- 

 tially electrical — electrostatic — in nature. This suggests at 

 once an electrical structure for molecules themselves, i.e. 

 that they consist of positively and negatively charged particles 



