ill determining Gaseous Combination. 357 



'fc> 



escaping unburned. These actions are exhibited in the first 

 experiment. But if the heat be raised to the degree at which 

 oxide of copper is reducible by hydrogen gas, the newly- 

 formed oxide yields its oxygen to hydrogen in the open air 

 just as in a closed tube, and becomes, as in the latter case, 

 incandescent. The continuance of the glow, after removing 

 the lamp, is due to a succession of alternate reductions and 

 reoxidations ; for the metal, at the moment of reduction, be- 

 ing at the temperature at which it can unite with the oxygen 

 of the air, is reconverted into oxide, which is again reduced 

 by the hydrogen ; the necessary temperature of the mass be- 

 ing sustained by other contiguous portions in the state of in- 

 candescence. It is therefore not metallic copper, but oxide of 

 copper, which induces the combustion of the hydrogen, and 

 the silent formation of water, hr further proof of this mode 

 of action, it may be added, that oxide of copper from the ni- 

 trate, exposed on a platina dish to a current of hydrogen with 

 free access of air, and gradually heated to below visible red- 

 ness, became incandescent, and continued to glow as long as 

 a stream of hydrogen was directed upon it. 



% II. Lead. 



Hydrate of lead thrown down by potassa from the acetate, 

 was heated in a glass tube, in contact with hydrogen gas, 

 until it was converted into a dark grey powder. This powder 

 was heated in the open air, and a stream of hydrogen was di- 

 rected upon its surface. It was notwithstanding converted into 

 protoxide, sometimes with, sometimes without, faint incan- 

 descence. On increasing the heat, the surface of the oxide 

 upon which the hydrogen impinged was again reduced, while 

 the remoter portions became pink and passed slowly into a 

 higher degree of oxidation. 



Since, then, recently reduced copper and lead, though rank- 

 ing among metals endowed with feeble affinities for oxygen, 

 exert a stronger affinity than hydrogen gas for that element, 

 it might be anticipated that the more oxidizable metals, and 

 especially such as are capable of decomposing water, would 

 a fortiori fail to induce direct gaseous union. 



% III. Cobalt. 



1. Oxalate of cobalt obtained from zaffre (by Dr. Thom- 

 son's process) was heated in a glass retort, witliout access of 

 air, till carbonic acid gas ceased to be evolved, and the me- 

 tallic cobalt was left in the form of a black powder. The co- 

 balt obtained by this process was not pyrophoric, nor did it 

 inflame a jet of hydrogen directed upon it at the existing tem- 

 perature. When heated to considerably below visible red- 



