J. H. Alexander on the Tension of Vapor of Water. 213 
ent, into one which will shew the pressure in atmospheres ;—a 
me ethod which, as it extricates the results from any dependence 
on a particular system of weights and measures and thus makes 
them generally applicable, is of course in any purely scientific in- 
vestigation, much to be preferred. It is obvious that such a scale 
of atmospheres or volumes, must start from the degree where the 
expansions and the tendency to expand (which is elasticity), so 
far as they are due to temperature, are null. The limit that we 
“vst found, then, of 105°:13 below 0° Fahr., is such a term; 
the distance between that and the other extremity of the 
ere or 317°°13, (which is the measure on the thermometer of 
one atmosphere, ) becomes the new denominator to replace the 
180° actually used for the pressure in inches of mercury. 
In fact, I had expected, in advance, to find the present mano- 
metric formula (as it may be termed) aah a barometric one, 
by putting it into this shape ; 
(eras (ie95) 
P=\3i7-13T \i695/ J - 
But this did not hold good. Applying it numerically, it results 
in giving, 
For 212° 18 pressure of 1:059 atmospheres, e equal to31- 68 inches. 
And for 322°°38, 6-263 187:37 
In both these instances, to agree with the original formula, the 
numbers representing the atmospheres should hayé been without 
fractions. ‘The excess, however, (as is visible,) goes on in a con- 
verging series, and by and by disappears altogether; the differ- 
ence then changing its sign. Even then, it is not much; and at 
high temperatures, the equation corresponds very nearly with the 
actual observations. For instance, comparing it with the exper- 
iments of Dulong and Arago, 
Temp. 335°: ae OR m pressure a iis atmose; by foreasia.1. 628 atm. 
ee: © ak 1-660 11-958 * 
Aeros as the object I we in view was not to find an equa- 
tion that merely fits any particular series of observations, or is ex- 
act only for the higher ranges of temperature, I abandoned this 
‘theoretical expression ; and preferred to deduce a formula for pres- 
sure in atmospheres from. the original one, in the ordinary ana- 
lytic way. ‘This results in the alternative expressions— 
& Anal t 561° coach of t 990 )’; ‘ 
| P=\3i713+ 1605 | = \317-13 + 2986-38 
either of which may be adopted, according as we prefer to re- 
tain in view the factor of the latent heat, or that of the expan- 
| sion at the unitary atmospheric pressure. In practice, the constant 
¥ fraction may be substituted by the number 0-33151. For the 
oe ara the similar constant was carried no farther than 
four pl decimals ; ; in this, where the unit of pressure is 
Seconp ath Vol. VI, No. 17.—Sept., 1848. 
