16 SATURATED STEAM, AND OTHER VAPORS. 



Assuming an equation of the form 

 A = A + J9f, 



Regnault calculated the constants from the values given for 100 and 195, 

 and gives the equation 



A = 606.5 + 0.305*. 



Wishing to see the effect of the varying value of the specific heat at low 

 temperatures, I recalculated the total heats given by experiment, by a method 

 resembling that used in recalculation of the heats of the liquid, and plotted 

 the results together with Hegnault's values uncorrected. The n < -al< -ulatcd 

 points were a little more regular than the original ones, and lay nearer (In- 

 line represented by the above equation. Especially did the recalculated 

 points for those experiments, for which the true mean specific heat of tin- 

 water in the calorimeter was nearly unity, lie near that line. It then-fun- 

 i rs that the equation represents our best knowledge of the total heat of 

 steam. 



For the Fahrenheit scale the equation becomes 



A = lOiU.7 + 0.305 (t 32). 



Heat of Vaporization. If the heat of the liquid be subtracted from the 

 total beat, the remainder is called the heat of vaporization, and is represented 

 by r, so that 



r = A q. 



Internal and External Latent Heat The heat of vaporization overcomes 

 external pressure, and changes the state from liquid to vapor at constant 

 temperature and pressure. Let the specific volume of the saturated vapor be 

 , and that of the liquid be rr, then the change of volume is s a- = u, on 

 passing from the liquid to the vaporous state. The external work is 



p(s - a) = pu, 

 and the corresponding amount of heat, or the external latent heat, is 



Ap(s <r) = Apu, 

 A being the reciprocal of the mechanical equivalent of heat. 



The heat required to do the disgregation work, or the internal latent 

 heat, is 



p = r A i 



Specific Volume and Density of Steam, On account of the great difllculty 

 of direct determination of the weight of saturated steam, it is customary to 

 calculate the specific volume of steam by aid of the following equation, 

 derived by the application of the principles of thermo-dynamics to the genersl 

 equation representing the properties of saturated vapor : 



r 1 

 AT'fy^ 

 dt 



