638 REGNAULT’S HYGROMETRICAL RESEARCHES. 
3. Condensing Hygrometers. 
Le Roy of Montpellier first proposed, with a view to deter- 
mine the hygrometric state of the air, to cool slowly some water 
inclosed in a vessel by successive additions of small quantities of 
ice until a deposit of dew began to form upon the sides of the 
vessel, The temperature which the water of the vessel indicates 
at this moment is that which the air would have completely 
saturated by the quantity of vapour in it. If ¢ represents the 
temperature of the ambient air, ¢' that which is indicated by a 
thermometer immersed in the water of the vessel, f and /’ the 
elastic forces of the aqueous vapour corresponding to those 
ah 
temperatures, 7 will be the fraction of saturation of the air. 
It is difficult, under most circumstances, to find ice to make 
this experiment. Some physicists have proposed to lower the 
temperature of the water in the vessel by dissolving in it certain 
salts, such as the nitrate of ammonia. But when the air is very 
dry and its temperature is high, it is often difficult to obtain in 
this way a sufficiently low temperature to produce the deposit 
of dew. 
The process of Le Roy received its first practical application by 
the construction of Daniell’s condensing hygrometer. It is well 
known that this instrument, fig. 6, consists of two bulbs A and B 
united by a large curved tube. The bulb A contains some ether, 
which rather more than half-fills it; a very delicate thermome- 
ter is arranged in the tube, so that its reservoir is in the centre 
of the bulb A and plunges into the upper strata of the liquid. 
A vacuum is completely made in this little apparatus before 
closing it by the lamp. The bulb B is enveloped in a piece of 
cambric, upon which the observer pours some zther, drop by 
drop, with a pipette. The evaporation of the «ther in the air 
produces a considerable cooling of the bulb B, which causes 
the distillation of the zther of the bulb A. The ether cools, 
and may descend below the temperature at which the air would 
be saturated by the quantity of vapour existing in it at the 
moment. Some dew will therefore be seen to form upon the 
bulb A. To render the first deposit of dew more apparent, the 
bulb A. is ordinarily constructed with a glass deeply coloured 
with cobalt, or it is coated with a brilliant gilt ring. 
The cooling of the ether in the bulb A principally takes place 
