6 Mr. T. R. Edmonds on the Law of Density of 



If £=0, we have 



PV=1, jr*=l> and L = l; 

 consequently 



that is, 



1 ««P V, . j_««P,Vo. 

 1 — t T or J — T , 



_ -0358 x 376 x 2116-4 x 26-36 ■ 



J — ma r — ioyy*7. 



536- 5 



The quantity J, whose value has thus been determined to be 

 1399' 7, is the mechanical equivalent in foot-pounds of a Centi- 

 grade unit of heat, the corresponding equivalent for a Fahren- 

 heit unit of heat being 777 foot-pounds. The value of such 

 mechanical equivalent as determined by Dr. Joule is 1389*6 

 for a Centigrade unit, or 772 for a Fahrenheit unit of heat. If 

 the constant V be assumed to represent 26*17 cubic feet instead 

 of 26*36 cubic feet, the resulting value of J will be identical 

 with the value adopted by Dr. Joule himself. 



It having been found that 



L= T T ° ° ?Yp~ n and J= - T ° & > 



it follows that 



L=¥Vp- n , 



an equation which represents the corresponding values of L and 

 PV when the latent heat, as well as the expansive force at the 

 absolute temperature a, whence / is measured, are taken equal 

 to unity. 



By means of the last equation and the formula for PV in 

 terms of log p, the latent heat may be expressed in terms of the 

 differential coefficient of the expansive force PV. The formula 

 mentioned gives on differentiation, 



d.logPV . ^.-rf.logPV w . 



d . P V 

 consequently, since ' =^.logPV and L = PVjo _ % 



d PV 



p£— = a,/*PV;T n = « / ,«L^-80840L. 



That is, the latent heat of saturated steam (relative to water at 

 the same temperature) is equal to the differential coefficient of 

 the expansive force PV, divided by « // a( = -80840). 



By means of the differential coefficient of log PV above given, 



