182 ME. MACQFOEJf EAXKINE ON THE THEEMO-DTNAZ^nC THEOEY 
their difference, which consequently becomes simply 
and that because J is constant during the evaporation, the value of the above mtegral 
ig where v denotes the excess of the volume of the va'pour above that of the liquid. 
dP 
or the incTease of volume in the act of evapovation ; so that, finall} , the latent heat of 
evaporation under the pressure p, in units of mechanical energy, is 
( 5 -) 
Now the relation between the pressure p and boiling-point t for water and various 
other fluids has been ascertained by the experiments of M. Eegxault, and expressed 
in tables and formulse ; and for water, the relation between the boiling-point t and latent 
heat of evaporation H, has also been ascertained by M. Eegxault’s experiments in 
ordinary units of heat, which by the aid of “Joule’s equivalent” can be reduced to 
units of energy ; consequently the bstceease of volume of oxe pouxd of watee ix 
evapoeatixg is given by the equation 
The volume of a given weight of liquid water at all ordinary pressures is so small, as 
compared with that of the same weight of steam, that for practical pui-poses r may 
without material error be considered as representing the volume of one pound of steam. 
The latent heat of evaporation of one pound of water at the boiling-point f m foot- 
pounds of energy, is given with sufficient accuracy by the formulae 
Il=a-bt, ('•) 
where «=1109550 foot-pounds, 
and b= 540’4 foot-pounds per degree of Faheexheit. 
The pressure of saturated steam is given with great exactness, at all boiling-points at 
which it has yet been measured, by the formula 
com. logy?=A — 
B C 
t P 
(S-) 
(first published in the Edinburgh Philosophical Journal, 1849). The values of the con- 
stants A, B, C, given as revised, in the Philosophical Magazine for December 1854. are 
as follows : — 
For pounds on the square foot, A= 8-2591. 
For pounds on the square inch, A= 6-1007. 
For absolute temperatures in degrees of Faheexheit, 
logB=3-43642; log C=5-59873. 
