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ARTICLE XXI. 
Hygrometrical Researches. By M.V. REGNAULT. 
[From the Comptes Rendus, T. xx. pp. 1127 and 1220 for April 1845.] 
THE general problem of hygrometry consists in determining 
the quantity of aqueous vapour existing at any time in a given 
volume of air, and the relation between this quantity and that 
which the air would contain if it were perfectly saturated. The 
different methods which have been proposed to attain this ob- 
ject are of two kinds. 
The first are purely chemical: they consist in absorbing, by 
means of substances which have a great affinity for water, the 
vapour contained in a certain volume of air, and in determining 
its weight by the balance. The others are founded on the ob- 
servation of certain physical phenomena, such as the greater or 
lesser elongation which substances of organic origin experience in 
air more or less near to the state of saturation, or on the deter- 
mination of the temperature to which the air must be lowered 
in order to be saturated by the quantity of humidity which it 
holds. 
All these methods presuppose the exact knowledge of certain 
physical laws and of several numerical data, as— 
1. An exact table of the elastic forces of aqueous vapour in 
saturated air for all atmospherical temperatures ; 
2. The density of the aqueous vapour in relation to the air 
taken under the same circumstances, when the air is saturated 
with the vapour ; 
3. The density of this same vapour when the air is saturated 
in a greater or less degree with it. 
The memoir which I have the honour to lay before the Aca- 
demy contains the results of numerous experiments in which I 
have been engaged for several years, on that branch of general 
physics, which, notwithstanding the efforts of several distin- 
guished philosophers, still presents many points of uncertainty, 
I shall divide this investigation into two parts: in the first, I 
shall direct my attention to the fundamental data which I have 
just enumerated; and the second part will be devoted to the 
examination of the hygrometrical processes. 
