468 
may be mentioned that, when the wire was immersed in 
distilled water which prevented its temperature from sensi- 
bly rising, the intensity of the current was almost exactly 
twice as great as when the wire was allowed to become 
heated, in atmospheric air at the ordinary pressure. 
The appearances presented by the platina wire corres- 
ponded with the foregoing results. In atmospheric air, it 
exhibited a bright red heat; in the muriatic and sulphurous 
acid gases, the redness was distinctly a shade brighter; in 
cyanogen, carbonic oxide, and hydrogen, there was no sen- 
sible difference ; in carbonic acid, oxygen, and the deutox- 
ide of nitrogen, the wire, so far as the eye could judge, ap- 
peared rather duller than in air; while, in olefiant gas and 
ammonia, it was only raised to a very obscure red heat, and 
in hydrogen, no redness whatever was visible, even in com- 
plete darkness. This method may, it is obvious, be ex- 
tended to vapours; and, from some trials made with them, 
it appeared that the cooling powers of the vapours of alco- 
hol and ether are considerably greater than the cooling 
power of air, and that of steam very slightly greater. On 
the other hand, the cooling power of all gases diminishes as 
they become rarefied ; so much so, that the platina wire used 
in the preceding experiments reached in vacuum nearly its 
point of fusion, while, at the same time, the intensity of the 
current considerably diminished. 
The gases may be conveniently arranged into the following 
groups, in reference to their cooling powers; and it will be 
found, on inspecting the table, that those arranged in each 
group differ little in this property from each other :— 
Group I. Gases whose cooling power is less than that 
of atmospheric air :—sulphurous acid, muriatic acid. 
Group II. Gases whose cooling power is nearly the 
same :—nitrogen, carbonic oxide, cyanogen, carbonic acid, 
deutoxide of nitrogen, protoxide of nitrogen, oxygen, va- 
pour of water. 
