At Hygrometrical Experimeitts. 
eonclusion that nitrogen had been decomposed, and had 
formed hydrogen and oxygen ; and J cannot see how it is 
possible to reconcile, in any other manner, the apparently 
accurate analyses of ammoniacal gas with the exhibition of 
oxygen in these experiments. 
[To be continued] 
IX. Description of an Almometer, and an Account of some 
Photometric, Hygrometric, and Hyzroscopical Experi- 
ments. By Professor Lustie, of Edinburgh*. 
I, appeats that steam, however formed, has probably double 
the elasticity of common air, and under the same pressure 
would occupy twice as much space. In uniting with that 
fluid it must hence communicate an expansion propor- 
tionate to the quantity dissolved, or to the portion of mois- 
ture required for the complete saturation of the air. This 
law suggests the principle of the hygrometer. But the pro- 
cess of evaporation is still misconceived. The depression 
of temperature which always accompanies it, has been 
hastily supposed to be proportional to the rate at which the 
moisture is dissipated, and to be therefore augmented by 
every circumstance that can accelerate-this effect. But if 
water be exposed fo a current of air it will cool to a certain 
point, and there its temperature will remain stationary ; the 
rapidity of the current may hasten the equilibrium, yet 
the degree of cold induced will be thé same, as the medium 
which supports the vapour furnishes the heat necessary to 
its formation. In fact, after the water has been oncé 
cooled down, each portion of the ambient air which comes 
to touch the evaporating surface must, from its contact 
with a substance so much «denser than itself, be likewise 
cooled down to the same standard, and must hence com- 
miunicate to the liquid its surplus heat. Every shell of air 
that in suecession eneircles the humid mass, while it ab- 
sorbs along with the moisture which it dissolves the mea- 
sure of heat necessary to convert this into steam, does at 
the same instant thus deposit an equal measure of its own 
heat on the chill, exbaling surface. The abstraction of 
heat by vaporization on the one hand, and on the other its 
deposition at the surface of contact, are theresore opposite 
contemporaneous acts, which soon produce a mutual ba- 
lance, and thereafter the temperature induced continues 
* Abstracted from “ Ashort Account of Experiments and Instruments 
depending on the Relations of Air, Heat, and Moisture.” j 
without 
