5 & 
212 J. H: Alexander on the Tension of Vapor of Water. 
bers corresponding to any linear scale. Thus, for instance, sub- 
stituting Centigrade degrees for those of Fahrenheit, but carrying 
the denominator of the first term down to the degree at which 
the pressure becomes, by the formula, zero (which may be pre- 
sumed to correspond to an absolute negation of heat, and which 
in fact has to be used with the present formula when it is in- 
tended to give the pressure in atmospheres) ; we obtain pressures 
expressed in French metres and, through a range of several 
atmospheres, but little discordant with the results of experiment. 
In using Centigrade degrees, however, and transforming the equa~ 
tion so as to express atmospheres, the index (6°) gradually diver- 
ges from regular multiples; serving to shew what we might oth- 
erwise conjecture, that such index is not based upon any general 
relation in nature. . It was, therefore, of less interest for me to 
weary myself with comparisons of other thermometric and linear 
scales; it is enough that the formula affords a remarkable coinci- 
dence in its own terms with the measures recognized among our- 
selves, 
It is readily seen that the first term is positive as far as 0° of 
Fahrenheit ; for temperatures lower than that, it becomes nega- 
tive ; and there is a point, of course, Where the negative value of 
the first term equals the constant positive value of the second, 
and the pressure, therefore, as was said jist now, becomes itself 
zero. This point occurs at — 1059-13; of which it is enough to 
say, that it is not very far from the lowest degree of heat yet 
produced, and that long before it is reached the mercurial ther- 
mometer becomes useless. Whether there is in the theory of na- 
ture (for it is admitted that there is ndét in fact) such a point as 
that of the absolute privation of heat, and if so, how it should 
be reckoned and where placed,—are questions which, although 
kindred to the matter in hand, are not necessary to the elucida- 
tion 
(as it was to the method of interpolation of Dr. Dalton) that it 
lere it should not stand, it 
iy att yes hold good against the factors without prejudice to 
formula was not so very discordant from the results of experi- 
ment, I had the less motive for modifying or transforming it into 
a nearer agreement at this unusual temperature. 
The determination of a limit of this sort, whether real or as- 
sumed, is necessary in. converting easily a formula like the pres- 
ie 
