22 Mr. Grove on the Decomposition of Water by Heat. 



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 luminous 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 atmo- 

 spheric air, at the same temperature and pressure ; the ther- 

 mometer rose 1^° 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 differ- 

 ences shown in the above table ; there appears a general ten- 

 dency 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 pa- 

 rallel does not hold ; sound is transmitted imperfectly through 

 rarefied 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 ; hydrogen, 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 exposure 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, occa- 

 sionally having an inch depth of 

 oil on the surface (see fig. 2t); 

 when over mercury, and a long- 

 continued exposure was required, 

 a bent tube was employed, as at 

 fig. 3, the closed end being im- 

 mersed in water or oil, to prevent 

 the fusion of the glass which 

 would otherwise have ensued. 



* Transactions of the Cambridge Philosophical Society, vol. i. p.267' 

 t In this and in figs. 3 and 5, the lines leading from the platinum loop 

 to tfie mercury cups represent copper wires. 



Fig. 2. 



