308 EXPERIMENTAL INVESTIGATIONS. 



best shown by including a voltameter in the circuit. Davy 

 found that the conducting power of a wire diminished in pro- 

 portion to the degree to which it was heated : assuming the 

 accuracy of this position, the amount of gas in the voltameter 

 would be inverse to the intensity of ignition in the wire. 

 The following is the result I obtained with different gases, 

 employing the same battery (the nitric acid combination at 

 its most constant period), the same wire, and the same 

 vessel : 



Cubic inches of Gas evolved in 

 Gases surrounding the wire. the voltameter, per minute. 



Hydrogen 77 



Olefiant gas 7*0 



Carbonic oxide 6'6 



Carbonic acid 6'6 



Oxygen 6*5 



Compressed air, 2 atmospheres . 6*5 



Nitrogen 6*4 



Atmospheric air . . . ... . 6*4 



Rarefied air. 6*3 



Chlorine 6'i 



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 7-2 m fi ye niinutes in the hydrogen, and 1 5 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 com- 

 bustible gases, but the facts are far too few to found a gene- 

 ralisation. I was at first inclined to regard the difference of 

 effect in hydrogen as analogous to the peculiarity mentioned 



