210 M. A. Hitter's Contributions to the Study 



per square metre, the melting-point rises from the isothermal 

 for t = to that for 



^^^^■OQIU ( 8 ) 



Since at this temperature the pressure of saturated vapour is 

 also 62*58 kilograms weight per square metre (or 0*006 atmo- 

 sphere), it follows that the principal edge coincides with the 

 straight segment of the isothermal for 0°*00744 C. and of the 

 isobar for 0*006 atmosphere pressure. 



The rime-edge EK (fig. l>) can be conceived as the line 

 along which the direct passage of ice into the gaseous state 

 begins. In applying equation (1) to the sublimation of ice, 

 we have to put r + l'm place of r, and u — U in place of u ; then 

 for the relation between the pressure and the sublimation-point 

 of ice we have the differential equation 



d P - r + l (q\ 



dT~A(u-u)T W 



In the isothermal for 0° C. this differential coefficient takes the 



value 



dp_ 424 x (606-5 +80) g. nfl n() s 



dT (210-66 -0-00009) X 27*3 > ' ' ^ ' 



frcm which the corresponding value for the isothermal for 

 t = 0*00744 differs by an insignificant quantity. For the angle 

 marked cd in fig. 10 we therefore have 



tano> = 5'06, or o) = 78°50 / (11) 



The angle co is thus greater than the angle <f> by 1° 25'*. 



Hence it follows that the principal edge is & prominent edge 

 in the part K L, but a receding edge in the part J K. 



We shall naturally find values for the angles (/>, yjr, co essen- 

 tially different frcm the above when we employ a different 

 unit in measuring either p or T in the construction of the 

 temperature-surface. If, for instance, we choose one atmo- 

 sphere as the unit of pressure, denoting by n the pressure in 

 atmospheres, then we have 



dp=10333dn, (12) 



and we obtain with this system of units the following equa- 

 tions for the angles: — 



tan<£= 0*000434, or <j>= 0° V 30"; . . (13) 

 tan^=133*6, orf=89°34'; . . . (14) 



tanw= 0*00049, or <o= 0° 1' 41". . . (15) 



* Compare Kircblioff, Pcgg. Ann. ciii. p. 206. 



