112 Prof. W. C. McC. Lewis on the Relation of the 



In the first place all ordinary liquids and vapours (with 

 the exception o£ oxygen and nitric oxide) are diamagnetic. 

 The susceptibility k for diamagnetic substances — bismuth 

 excepted — is taken to be independent of temperature (N. 

 Campbell, 'Modern Electrical Theory,' p. 118, 2nd ed.). 



Pascal, I. c. states, however, as an experimental result*, 

 that the product kV, where V is the specific volume of the 

 diamagnetic substance considered, is constant, independent 

 of the temperature and of the physical state. Denoting this 



constant by r ft it follows that =—- = ~ s and since r is 



a negative term ^r* is really positive, i. e. fi approximates 



to unity as the volume increases. 



As regards the temperature coefficient of the internal 



pressure it. since it— — - it follows that 



"dir _ a ~d/ji 2a ~dv 

 dT"~/^ 2 §T~/u; 3 §T' 



and since cV ^ / in d^ 2a ^v- 



55=0 (nearly), 5^=--,^ 



Hence 1 Btt 2 ~dv 1 ^ v 



— ^tt = vT' = ~- a ? where a = - =-= , 



7T Bl V qL V C)T 



the coefficient of expansion of the liquid. Of course the 

 same result is obtained from van der Waals' original ex- 

 pression. In connexion with the above relation, attention 

 may be called to the fact that Davies (Phil. Mag. [6] xxiv. 

 p. 421, 1912) finds just half the above value, which in turn 



leads to the relation 7r=-or7r = — . Davies employed 



v fiv r J ' ' 



the general van der Waals* type of equation, viz. 

 (p + 7r)(v — b) =RT, his considerations being based mainly 

 upon the Cailletet-Mathias law of the rectilinear diameter. 

 Davies is therefore dealing with a liquid Under the pressure 

 of its own saturated vapour, but this of course does not 

 account for the difference in the two values. 



Returning to Pascal's relation, viz. kv = r , it follows 



that — 7-^rnr = ~ =^7ii=«, a relation which could be tested 

 k ol v oi 



* I am unable to judge the degree of accuracy of this remarkable 

 relation h V = constant, as Pascal simply states it in the above papers as a , 

 fact without giving further details. 



