Valuation in the Pressure of Saturated Vapours. 81 



Mercury. 



22. Mercury belongs to the small number of substances for 

 which the maximum or minimum of x has a negative value. 

 In this case c — Ci must inevitably be < 0. Up to now, with 

 all the substances investigated, it has been found that c > c x . 

 There is, however, no reason for affirming that this aspect of 

 inequality is necessary ; at least, in the mechanical theory of 

 heat there is no indication one way or another. It is possible 

 to imagine a substance which would at certain temperatures 

 have a greater specific heat in a liquid than a gaseous state. 

 The inequality c<c ± induces the conclusion that the heat of 

 vaporization increases with a rise of temperature when the 

 vapour follows Boyle's and Gay-Lussac's laws. As a rule a 

 converse phenomenon is observed, i. e. there is a decrease. 

 Only ethyl alcohol, according to Regnault's experiments, 

 offers a remarkable exception ; the heat of vaporization in- 

 creases as the temperature rises from 0° to 20°, and then 

 follows the general law, i. e. it decreases. If x had a maximum 

 or minimum value between 0° and 20°, then the inequality 

 c— Ci<0 would be allowable between these temperatures. 



1st Method. 



t 250° .270° 280° 290° 300° 



x -2-9022 -1-8252 -5-2979 -4-8474 -4-6333 



t 310° 320° 380° 430° 



x -4-5703 -2-6758 -3-4592 +0-1945 



Although these figures present irregularities, still the mini- 

 mum, without doubt, occurs between 280° and 300°. It may 

 be said that the minimum value of x is equal to the arith- 

 metical mean of its three values corresponding to temperatures 

 280°, 290°, and 300°, namely 4*9262, and lies opposite 290°. 

 Hence 



c- Cl = -4-9262 x ^ = -0-049262. 



According to KupfFer, c = 0*0335 ; c ± has not been measured, 

 but, on the basis of what has been said above, 



d=0-0335 + 0-049262 = 0-0828. 



Further, we find 



?/ = 7092-6, 



r =7092-6x ^o=70-92. 



According to Berthelot, r at the atmospheric pressure =70*0. 

 Phil Mag. S. 5. Vol. 37. No. 224. Jan. 1894. G 



