J. H. Alecander on the Tension of Vapor of Water. 211 
steam; which repulsive force may be assumed to be at least a 
function of the elasticity of the vapor. And as such repulsive 
force takes effect in part by expanding the medium vaporized ; 
and the greater such expansion, the less will be the remaining 
elasticity ; it follows that the pressure of steam in the ordinary 
state of an atmosphere, must be also inversely as its increased 
volume. ‘This increased volume may be taken, from the experi- 
ments of Gay-Lussae, at 1695 times that of water at its greatest 
density. And the latent heat of steam is ‘generally admitted as 
990° F'.; which number results from the experiments of Clement 
and Desormes, is not far from the mean of several other observ- — 
ers, and will probably require a very small modification only to 
be identical with the deduction from a strict theory of volumes 
applied to vapors generally, in the mechanical relation of the ob- 
owe effects of heat rin such age and the mcg eae: 
t 
So far as we have gone, then, the pressure of ciation must wily 
( Se eamee It is here that the empiricism comes in, and dic- 
tates the numerical power to which this ratio must be involved, 
in order to harmonize. the pr 3 ce of. the elasticities with the 
series of the temperature. That the simple ratio will not present 
_ such harmony, is manifest; for, that wotild be the measurement, 
by a lineal scale of equal. parts, of central forces which, acting 
upon volumes, $y 22 naa be suppos ed to be in the duplicate ra- 
tio of the cube. ratio, equivalent to the sixth power, is 
in fact what has rides ; eh; and complete sate becomes 
then, calling p the pressure : : 
: ( t son) % 
-. P=\is0* 1695 
This index, however plausible wpon reflection the reason to 
justify its adoption might have appeared, was no doubt suggested 
too, even unconsciously, from the recollection that it had before 
(though with different factors) served’ both Young and 'Tredgold 
and perhaps others, in approximating the’ results of experiments. 
not be to occur upon the use of any other scale t 
that of F. heit; unless, indeéd, one should adopt the fancy 
of Mr. Woolf, who supposed that lie had discovered an i 
ate numerical relation between the atmospheric pressure and the 
pound avoirdupois, in which case the English inch ra alt =. 
claim to be among Natural dimensions. It is, of course, qui 
possible, by a little artifice among the terms of a formula of this 
shape and retaining the same index, to producesa‘series of num- 
5 .. oe 
i sae eto eGR oo + 
oe 
