52 Mr. P. J. Hartog and Dr. J. A. Harker on 



The quantities J and s are by no means easy to 

 measure, nor have we space to discuss the values obtained 

 for them by different observers. There is, however, reason 

 to believe that the value for J given by Griffiths (Phil. 

 Trans., 1893, P- 493)» vlz "> 4' I 94X IO? C.G.S. units, is within 

 two- of the truth. The experiments of Perot (Ann. Chim. 

 Phys. [6] XIII., p. 159) were performed with extreme care, 

 and we accept his value for the specific volume of steam at 

 99"6o°C., namely, 1657CC. 



We have used two formulae to calculate dp\dT from 

 Regnault's experiments, which give results differing by 

 less than 1 per thousand. That of Moritz gives dpjdT = 

 3'58574Xio 4 C.G.S. units; that of Roche gives dj>/dT = 

 3-58318 C.G.S. units, taking ^-=980-67.* 



If we calculate out L with the values given, we find that 

 with Moritz's number L= 527*54 calories, with Roche's 

 number, L= 527*16 calories, for the temperature 99*60 C. 

 (which we choose because Perot's determination was made 

 at that temperature). At ioo° the value of L would be 

 somewhat less (about half a unit, if Regnault's interpolation 

 formula is approximately correct.) 



These numbers agree well with the one obtained by us, 

 viz., 524*8, but at the same time, we should hesitate to 

 regard this confirmation as conclusive. 



* Moritz's formula, quoted by Wiillner, Lehrbuch der Experimental- Physik, 

 Vol. IV., p. 683, is as follows : — 



log. 10 p = a J rb* t —c$ t 

 where p = pressure in mm of mercury, t = temperature centigrade. 



log. 10 a = _ -006864937 



log. 10 3=i-996725536 



log. 10 ^=2-131990711 



log. 10 ^ = 0-611740767 



* -=47393707- 



Roche's formula, quoted by Him, Thcorie Mecanique de la C/ialew, 

 T. L, pp. 323, 325, is as follows :— 



dp _ 0*0909 36948^ 



'dT~ [i+o-oo49528~i67(20 + /) -2 J 



