REGNAULT’S HYGROMETRICAL RESEARCHES. 611 
quisite correction, if it be found that these tensions are really 
somewhat greater than those which exist in the air. 
Of the Density of Aqueous Vapour. 
What are the densities of aqueous vapour in vacuo and in 
the air, at the state of saturation or of non-saturation, for dif- 
ferent temperatures and under different pressures ? 
It is generally admitted that it is sufficient to determine by 
direct experiment the density of aqueous vapour for one tem- 
perature and pressure, and that the densities of this vapour may 
be subsequently calculated for all the other temperatures and 
pressures, by applying Mariotte’s law, and that of the uniform 
dilatation of gases. Now, experiment has shown that these laws 
are not verified, in most gases, even when these are far removed 
from their point of liquefaction. Consequently it is to he feared 
that these laws are not perfectly accurate for aqueous vapour, 
especially at the state of saturation, that is to say, at the very 
point of its liquefaction. 
A theoretical value of the density of aqueous vapour may be 
obtained by applying to this substance M. Gay-Lussac’s law on 
the composition of gases. 
Thus, 2 volumes of hydrogen weigh . . 01382 
1 volume of oxygen weighs . . . 11055 
2 volumes of aqueous vapour . . 1°2437 
Hence the theoretical density of aqueous vapour is 0°6219. 
Now it is necessary to ascertain whether the densities of aque- 
ous vapour under these different circumstances may be deduced 
by calculation from this theoretical density. It is impossible to 
decide this question with the means which science at present 
possesses. We have, in fact, a great number of determinations of 
the density of aqueous vapour, made under the most varied cir- 
cumstances by a great number of experimenters; but they pre- 
sent such extraordinary discrepancies, that it is impossible to 
ascertain the truth. They may be judged of by the following 
citations * :— 
o) 
Watt oe ee ee LOO OG 0°6334 
eyes te oe ean, 0°6666 
econ es ea, 0°7000 
Sadseure’ 5°94 07409 
* These quotations are extracted from the memoir of M. Schmedding, which 
wil be considered hereafter. 
VOL. IV. PART XVI. ox 
