of Edinburgh^ Session 1864-65. 451 
pressure of saturation (a being a constant to be deduced from one 
experiment for each fluid) ; — 
hyp. log p = a 
liyp. log 
(c —c)c—c 
(A) 
When c" is constant (as is approximately the case in some instances) 
the preceding equation becomes 
h c" — c' 
hyp. log p=a- — hyp. log t . (B.) 
[c —c)t c — c 
The pressures of various vapours, as calculated on the supposition 
of their being perfectly gaseous by means of the preceding equa- 
tions, are compared with their actual pressures ; the general result 
being, that when the vapours are rare, the differences are small, 
and that when the densities increase, the differences increase. For 
example, in the case of steam, the pressures calculated by equation 
B agree very closely with the actual pressures from 0° to 160° 
Cent. ; but above the latter temperature the difference gradually 
becomes considerable, and at 220° Cent, is about one-fiftieth part 
of the whole pressure. At 0° Cent. 1 pound of saturated steam 
occupies 3400 cubic feet ; at 160° Cent, about 5 cubic feet ; and at 
228° Cent, about 1-4 cubic foot. 
The author also makes some comparisons between the actual 
volumes of saturated vapours at given boiling-points, and the calcu- 
lated volumes which they would fill if they were perfectly gaseous ; 
and also between the actual latent heat of evaporation, and the 
calculated latent heat of perfect gasefication. The general results 
are in accordance with what is already known, — viz., that the 
actual volumes of vapours are less than those corresponding to the 
perfectly gaseous state, and the actual latent heat of evaporation 
less than the latent heat of gasefication ; and the author further 
points out that the differences in the case of steam increase nearly 
as the absolute temperature. 
