538 



Mr. J. P. Dalton on the Specific 



From fig. 1 the following table of inversion points was 

 derived : — 



j 



Inversion Temperatures. 



Too- 









r 



e. 2 . 



1-00 



o-ooo 



1-00 



1-02 



0-07 



0-995 



104 



0-14 



0-99 



1-06 



0-195 



0-985 



1-08 



0-25 



0-98 



MO 



0-305 



0-970 



112 



0-36 



0-96 



1-14 



0-42 



0935 



]-16 



0-48 



0905 



1-18 



0-555 



0-86 



1-20 



0-68 



0-765 



1-202 



0725 



0-725 



Besides the fact that there must be two inversion points or 

 none at all (except in the limiting case when the two coincide), 

 it is also worth noting that the region to the right of the 

 curve is the region of negative specific heats, to the left, i. e. 

 enclosed by the curve, of positive; while on the curve itself 

 the specific heat is zero. Hence 



(1) Saturated vapours of substances whose 7 00 *> 1*202 have 

 always a negative specific heat ; 



(2) Saturated vapours of substances whose 7^^1*202 have 



a specific heat that, as the temperature rises, is first 

 negative, then positive, and finally negative again ; 



(3) In the limiting case when 7^ =1*202 the two inversion 



points coincide ; the specific heat is then always 

 negative, except at one point when it becomes zero. 

 Kamerlingh Onnes*, in a paper dealing with the reduced 



i/r-surface, makes use of the equation (which is not quite 



exact) 



6 da 

 codO 



1 

 7-1' 



for the determination of inversion temperatures according to 

 van der Waals's equation, and finds that according to it the 

 limit of y^ is 1*195 if inversion is to be possible. He does 

 not seem to note, however, that every saturated vapour that 

 changes its sign once, must do so a second time, and his 

 division of substances into classes does not appear to be 

 entirely in accordance with the above results. 



* Kamerlingh Onnes, Comm. Leid. No. 66 (1900). 



