UOiO. . . {(}
To a pressure-increment therefore of 1380545 kilograms
weight per square metre (or 133*6 atmospheres) corresponds
a lowering of the melting-point by one degree C, if the dif-
ferential coefficient does not alter in value for this change of
temperature t» It follows that near the principal edge the
band of the surface which lies between the frost- and melting-
edges, and represents the mixture of ice and water (fig. 9 J,
makes a very small angle with the horizontal plane, since an
extremely small lowering of the isothermal for the melting-
point corresponds to a very considerable increment of pressure.
The angle marked y\r in fig. 10 differs therefore only very little
from a right angle.
To a diminution of the pressure by the weight of 1 kilogram
per square metre would, by equation (7), correspond a rise
of the melting-point by tswq oTs degree. When, therefore, the
pressure diminishes from 10333 to 62*58 kilograms weight
* Wiillner, Experimentalphysik, 2nd ed. iii. p. 548.
t Clausius, Mechanische Wdrmetheorie, 2nd ed. i. p. 173. [This result
was first shown by Professor J. Thomson. — Tb,.] a
Phil. Mag. S. 5. Vol. 5. No. 30. March 1878. P
210 M. A. Hitter's Contributions to the Study
per square metre, the melting-point rises from the isothermal
for t = 0 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
dP - 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 denotes a function of q, or
4>=J q(l-n)^dp, (2c)
in which p, the pressure of the vapour above the salt- solution,
is likewise a function of q, we get
3= — J dco .