MR. GROVE ON THE DECOMPOSITION OF WATER BY HEAT. 
3 
To ascertain the relation between the amount of radiant heat generated by the 
same battery and wire in gases which presented striking differences as to the lumi- 
nous effects of the platinum wire, an apparatus was prepared in which the bulb of a 
thermometer was retained at a certain distance from the coil of wire ignited by a 
battery of four cells, and exposed, first, to an atmosphere of hydrogen, and then to 
one of atmospheric air, at the same temperature and pressure ; the thermometer rose 
7^° in five minutes in the hydrogen, and 15° in the air in the same time. Both the 
heating and luminous effects appear therefore to be greater in atmospheric air 
than in hydrogen. I cannot satisfactorily account for the differences shown in the 
above table ; there appears a general tendency to greater ignition in the electro- 
negative than in the combustible gases, but the facts are far too few to found a 
generalization. I was at first inclined to regard the difference of effect in hydrogen 
as analogous to the peculiarity mentioned by Leslie* respecting its convection of 
sound, but the parallel does not hold ; sound is transmitted imperfectly through rare- 
fied air, and also through hydrogen ; on the contrary, the heat of the ignited wire is 
most intense in the former, and least so in the latter ; the heat is also very much 
reduced in intensity in the compounds of hydrogen, ammonia and olefiant gas, or 
even by a small admixture of hydrogen with another gas, such as nitrogen ; hydro- 
gen, therefore, appears to have a peculiar and specific action in this respect. 
I now pass to the consideration of the effects of the ignited wire on different gases. 
The ignition was in every case raised to the fullest extent, and the gases after expo- 
sure to it were carefully cooled down to their original temperature. 
When the experiments were made over water, the whole eudiometer was immersed 
in a vessel of distilled water, occasionally having an inch depth of oil on the surface 
(see fig. 2-f'-) ; when over mercury, and a long-continued exposure was required, a bent 
tube was employed, as at fig. 3, the closed end being immersed in water or oil, to 
prevent the fusion of the glass which would otherwise have ensued. 
The tubes are much more easily preserved from cracking, and the ignition better 
kept up with oil on the exterior than with water, but as in many of these experiments 
I might have been considerably misled by a crack in the glass, or a bad sealing of 
the wire, allowing a portion of oil to enter the tube, I used water in the greater 
number of them until I was assured of the phenomena. 
The apparatus, fig. 3, is superior in one respect to fig. 2, even for experiments over 
water, as the wire being situate below the volume of gas, the circulation is more 
rapid. This object may also be effected by employing the form of eudiometer, fig. 4, 
in which the loop of wire is near the centre of the tube, so as to be just above the 
surface of water in the tube ; there are, however, some difficulties of manipulation 
with this form, which render it practically of less value than fig. 1. 
* Transactions of the Cambridge Philosophical Society, vol. i. p. 267. 
f In this and in figs. 3 and 5, the lines leading from the platinum loop to the mercury cups represent copper 
wires. 
