210 «J. H. Alexander 3, the Tension of Vapor of Water. 
Arr. XIX.—On a New empirical Formula for ascertaining 
the Tension of Vapor of Water, at any Temperature; by 
J. H. Avexanver, Esq. . 
Tue formula which the following memoir is intended to ex- 
pose, is called new ; because, fo the best of my knowledge, it has 
never been used or suggested hitherto. It is also rightly termed 
empirical, in so far as it is not susceptible of geometrical demon- 
stration, but thus far only ; since, in point of fact, it was derived 
entirely from considerations a priori and independent of any ex- 
periment or interpolation. Of course, it was compared as soon as 
ible with the temperature corresponding to the ordinary at- 
mospheric pressure; and after a satisfactory agreement had been 
found at this point, the aceord of the formula with observations 
through a range of experiment more extensive than has hitherto 
been ‘heluded in one and the same table, it is the principal aim 
of the presen to exhibit, after having shown in few words 
the reasonableness of the formula and its limits (or rather want of 
limits) in applieation ; a comparison, then, of the errors existing 
and admitted in several of the experimental series of the highest 
authority, with the differences developed at. the same epochs by 
the formula, will indicate the. ilitie favor of the latter, 
and the nature and amount of its reliability. . 
It is obvious'that the pressure of vapor or steam must be always 
in proportion to the absolute temperature at which it is produced. 
But as this temperattire is only observable relatively and upon an 
arbitrary scale; it is hecessaty, in ordér to obtain any thing like a 
ature. Or, what amounts to the same thing, the pressure of steam 
whose temperature is observed on any scale, is directly as the 
scale, calling ¢ the number of degrees at any temperature, the 
pressure of steam at that temperature must be’ proportionate to 
t . ' 
iso ‘48ain, the pressure of steam must be always directly as 
the absolute heat of conversion, or, as it is otherwise termed, the 
latent heat ; expressed, of course, in degrees of the scale assumed. 
For, the greater the number of degrees for such latent heat, *the 
greater also will be the repulsive force of the heat existing in the 
