Heat of Saturated Vapours. 541 



For normal saturated vapours, therefore, we might expect 



(1) a negative specific heat for y^ > 1*139 ; 



(2) a specific heat first negative, then positive, and finally- 



negative for 7,0 < 1*139 ; and 



(3) a negative specific heat vanishing at = O"835 for 



7oc = l-139. 



J. D. van derWaals* divides vapours into two classes: 

 (1) when 7<1*07 the sign of the specific heat is positive, 

 and (2) when "y>l*08 it is negative. When y does not 

 differ much from 1*08, nothing definite can be asserted about 

 the sign of the specific heat. 



Observations of actual inversion points are hardly sufficiently 

 numerous to enable a direct comparison to be made between 

 the foregoing results and experimental data. The few, 

 however, that are known are here given. 



yoo . observed. 9 X or 9 2 from Table. 



Acetone 1-132 (Wiedemann) 092 (Cazin). 0'91 



Benzene 1*129 ( do. )fO'67(do. ). 1 077 



I 0-70 (Dupre). J 

 Chloroform... 1139 ( do. ) 074 (do. do.). 0-825 

 1-110 (Miiller) 0-72 



The degree of correspondence is sufficient to make one 

 regret that further comparison is at present impossible ; it 

 justifies, however, a rejection of L. Natanson'sf "first approxi- 

 mation" that inversion occurs in various vapours at cor- 

 responding temperatures, and of his subsequent deduction of 

 an inversion point for hydrogen. 



Abnormal Substances. 



An attempt was made to extend the foregoing method to 

 abnormal substances, of which methyl alcohol was taken as 



an example. Data for the calculation of T/ ^ ) -^ were 



obtained from the researches of Ramsay and Young J. In 



this case, however, no data are available for the determination 

 of (f)(0). C v was therefore considered constant, and equal to 

 &>=„. The resulting • (y x , 6) curve was similar to that shown 

 in fig. 2, but the maximum value of y-o on the curve was 

 much lower than that for normal substances. It would seem, 



* van der Waals, Beibl. Bd. ii. p. 331 (1878). 

 t L. Natanson, Zeits. f. nhys. Chem. xvii. p. 276 (1895). 

 % Ramsay and Young-, Phil. Trans, clxxviii. A. pp. 57-93 (1887) and 

 Phil. Mag. [5] xxiv. pp. 196-212 (1887). 



