REGNAULT’S HYGROMETRICAL RESEARCHES. 613 
A German philosopher, M. Schmedding, published some years 
ago in Poggendorff’s Annalen (vol. xxvii. p. 40), an extensive 
and carefully executed research, in which he determined the 
weight of aqueous vapour in air in the state of saturation at dif- 
ferent atmospheric temperatures; and he arrived at the result, 
that the density of aqueous vapour in the saturated air in rela- 
tion to that of air taken under the same circumstances, increases 
in a very striking manner with the temperature. This variation 
may be judged of by the following numbers extracted from his 
memoir :— 
13°44 0-616 
16°25 0°621 
17°50 0-625 
18°75 0°627 
20-00 0°630 
21°25 0-632 
22°50 0°634 
23°75 0°643 
28°75 0°643 
37°50 0-640 
43°75 0:652 
The density of aqueous vapour varies therefore from 0°616 to 
0°652 between the temperatures of 13° C. and 44°C. We should 
therefore commit considerable errors by calculating the weight 
of the vapour which should be ina state of saturation in a cubic 
metre of air, with the theoretical density of which I have just 
spoken, and by applying Mariotte’s law and that of the uniform 
dilatation of the gases. 
I have made several experiments to decide this point, which 
is the basis of the theory of hygrometry. 
I first determined the density of aqueous vapour in vacuo, at 
the temperature of 100°, but under progressively decreasing 
pressures, in order to ascertain whether the vapour followed in 
this case the law of Mariottc. The apparatus which I employed 
consists of a balloon A (Plate VIII. fig. 1) of about 10 litres capa- 
city, bearing a brass mounting with a stopeock 7. This mount- 
ing is fixed to the neck of the balloon by means of red-lead 
cement, in the manner which I have described in my memoir on 
the density of gases*; it terminates in a curved brass tube. 
* Comptes Rendus, vol. xx. p. 975. 
2D 
