636 REGNAULT’S HYGROMETRICAL RESEARCHES, 
and adding to it a certain quantity of water, so as to bring it to 
the state of SO? + 4H?O; during this operation much heat is 
disengaged, and there is always water converted into a state of 
vapour, so that the liquor does not present the exact strength. 
Its composition is determined with the greatest care by chemical 
analysis. This liquor, the strength of which is accurately de- 
termined, is afterwards employed to form all the other solutions. 
These liquors may be preserved for a very long time in stop- 
pered bottles, and they may be employed to verify the gradua- 
tion of the instrument as often as is desired. 
The most essential precaution consists in placing the vessel 
containing the hygrometer in a spot where the temperature alters 
only very slowly, in order that the liquor may fairly present the 
temperature indicated by the thermometer. To effect this ob- 
ject, I place the glass in a wooden box having a small lateral 
door, which is only opened at the moment of the observation. 
It is easy to fit to the lid of the glass vessel, fig. 4, a metallic 
mounting which allows of making a complete vacuum by means 
of the air-pump. I have thus ascertained that the hygrometer in- 
dicates precisely the same degree in the air and in vacuo, when it 
is in presence of the same solution and at the same temperature ; 
but in vacuo its course is much more rapid, and a few minutes 
are sufficient for it to attain its stationary position, even when 
the fraction of saturation is very small. 
I have employed, in forming the table of the hygrometers, 
another process more complicated in execution, but which allows 
of obtaining the graduation of the hygrometer in a very short 
time, and of studying with great precision this graduation at 
different temperatures. 
A glass bell V V’, of about 15 litres in capacity, Pl. VIII. fig. 
5, rests upon a cast-iron pedestal. This pedestal has a groove, 
in which a very fusible cement is poured; the bell is thus closed 
hermetically from below. This bell carries a mounting A with 
several tubulatures. Into the central tubulature o a very accurate 
thermometer is fixed; to the second tubulature are soldered a 
first leaden tube ¢ d, which communicates with a barometrical 
manometer, and a second tube ef, which communicates with an 
air-pump. Lastly, the third tubulature has a stopcock 7; into 
this tubulature is cemented a small balloon containing water. 
The bell is arranged in a large glass vessel full of water. This 
vessel is itself placed in a cast-iron vessel full of water, which can 
