1873.] Gaseous, Liquid, and Solid States of Water. 31 



and M' to denote the corresponding rates for steam with ice at the 

 triple point, we have 



dp_ 



dt M 



The latent heat of evaporation of one pound of water at the freezing- 

 point (or triple point) into steam at the same temperature, as determined 

 by Eegnault, is 606-5 thermic units, the thermic unit being here taken 

 as the heat which would raise the temperature of one pound of water 

 one degree Centigrade ; and the latent heat of fusion of ice is about 78 or 

 79 of the same thermic units. Hence, though M and M' belong each to 

 a cubic foot of steam at the triple point, not to a pound mass of it, still 



M . 606 



the ratio 



Hence 



dp 



606 1 



dp 1 " 79 + 606 = Fl3* 

 ~dT 



This shows that for any small descent in temperature from the triple 

 point (where the pressure of steam with ice is the same as that of steam 

 with water), the pressure of steam with ice falls off 1'13 times as much 

 as does the pressure of steam with water. 



In submitting the quantitative calculation now given, I have preferred 

 to adopt the method proposed and developed by my brother rather than 

 that which I had myself previously devised, because his method is simpler, 

 and brings out the results more briefly by established principles from 

 existing experimental data. I may say, however, that the method devised 

 by myself was also a true method, and that I have -since worked it out to 

 its numerical results, and have found that these are quite in accordance 

 with those brought out by my brother. The two indeed may be regarded 

 as being essentially of the same nature ; and I think it unnecessary to 

 occupy space by giving any details of the method I planned and have 

 carried out. Its general character may be sufficiently gathered from the 

 concluding passages of the British-Association 1872 paper, as printed 

 in the Transactions of the Sections, Brighton Meeting. 



In order to discover whether the feature now developed by theoretical 

 considerations is to be found showing itself in any degree in the experi- 

 mental results of Eegnault on the pressures of steam at different tem- 

 peratures*, I have made careful examinations of his engraved curve 

 (plate viii. of his memoir), and of his empirical formulae adapted to 

 fit very closely to the results exhibited in that curve, and of his final 



* Regnault, " Des Forces Elastiques de la Vapeur d'Eau aux diffe'rentes Tempera- 

 tures," Me'moires de I'Acade'mie des Sciences, 1847. 



