REGNAULT’S HYGROMETRICAL RESEARCHES. 625 
periment on the elastic forces of aqueous vapour in a state of 
saturation in the air. 
However, it is seen that on taking as basis my table of the 
elastic forces of aqueous vapour in vacuo, and admitting that the 
density of the vapour is constantly equal to 0°622, that of the 
air under the same circumstances being 1, the calculated weight 
of the vapour cannot differ from the real quantity by more than 
a very small fraction, about one-hundredth. 
The experiments which I have described may be considered 
as confirming in a complete manner the accuracy of my table of 
the elastic forces of aqueous vapour at low temperatures. It is 
certain that on calculating these experiments with the old tables 
adopted by physicists, we obtain very conflicting results. The 
experiments which I have made on the density of aqueous va- 
pour, and which I have just described, were instituted several 
years ago, and on seeking to calculate them with the old tables, 
I found the necessity of making new determinations of the elastic 
forces of aqueous vapour at low temperatures, 
Part II. 
Processes employed to determine the fraction of Saturation of 
the Air. 
I shall distinguish four principal methods by means of which 
this determination may be performed :— 
1. The chemical method. 
2. The method founded upon the indications of hygrometers 
_ formed of organic substances which are lengthened by humidity. 
3. The method of the condensing hygrometer. 
4. The method of the psychrometer, namely, that which is 
founded upon the observation of the temperatures indicated 
simultaneously by two thermometers, one with a dry, the other 
with a moist bulb. 
1. Chemical Method. 
I have little to say on the chemical method, after the details 
into which I entered at page 617 and the following, to explain 
the experiments which I made with a view to determine the 
weight of the water which the air contains when it is saturated. 
When this method is employed to determine the quantity of 
moisture which exists in the air in a given place, it is necessary, 
by means of a long tube, to seek the air in this place and bring 
