Theory of Heat to the Study of Volatile Liquids. 483 



ference is only 3 calories, or ^bo of the real value. This result, 

 then, confirms and justifies our hypothesis. 



Equation IY. differs essentially from those which have been 

 proposed by MM. Clapeyron, Clausius, Zeuner, &c. by the 

 term c(t f —t), which has been totally neglected in the analysis 

 of the problems belonging to the investigation of latent heats. 



Before exhibiting the general Table of the results to which 

 this method of calculation leads, let us simplify equation IV. 

 by investigating the latent heat of liquids at the boiling-point. 

 For the temperature of ebullition the pressure P becomes the 

 same for all liquids ; and if we make t' converge towards t, 

 V will come indefinitely near to P ; therefore t' — t converges 



. <?) 



towards 0, and the fraction — becomes the derivate of the 



Z — t 



pressure in relation to the temperatures. Equation IY. can 

 then be written 



10333(274 + tf , . , e 

 • = - ^»ts-n — t^tt- x derivate of 



<?) 



l-293SEx274 t' -t 



To obtain this derivate, let us differentiate the equations of 

 the pressures expressed as functions of the temperatures in im- 

 mediate proximity to the boiling-point. In fig. 2 we have 

 represented the whole of the curves given by M. Pegnault. 

 The abscissas are reckoned from one 5 degrees to another above 

 and below the boiling-point ; and the ordinates are the corre- 

 sponding pressures. This curve shows clearly the result of 

 the differentiation — namely, that we arrive at a derivate which 

 is sensibly a constant multiplied by a simple function of the ab- 

 solute temperature for all liquids. The curve widens slightly 

 above and below the pressure of 760 millims. ; but in the im- 

 mediate vicinity of that pressure the same curve represents 

 very accurately all liquids. 



In the following Table we put the names of the volatile 

 liquids in the first column, in the second the temperatures of 

 the boiling-point, in the third the calculated latent heats, and 

 in the fourth the latent heats observed by M. Regnault and 

 given in his work. For very volatile liquids the derivate in- 

 creases very sensibly, as may be seen. We give here only a 

 few liquids from the general Table. 



2 K2 



