360 Dr. C. Henry's Experiments on the Action of Metals 



at the temperature of reduction. It then alternately absorbs 

 the oxygen of the air and yields it to the hydrogen, in the same 

 mode as a given portion of nitrous gas has been supposed to 

 act in repeatedly carrying the atmospheric oxygen to sul- 

 phurous acid in the process of making oil of vitriol. 



§ VI. Silver, 

 As a contrast to the results of the foregoing experiments, 

 oxide of silver, obtained from the nitrate by caustic potassa, 

 was heated on a platina dish, in the open airi and a current 

 of hydrogen made to flov^^ upon it. The oxide wvls instantly 

 reduced, and the pure metal underwent no further change, 

 but, when strongly heated, inflamed a jet of hydrogen, without 

 itself becoming incandescent. 



The property of inducing the union of hydrogen with oxy- 

 gen, at common temperatures, had been ascribed by former 

 inquirers to nickel as well as to the noble metals. It must now 

 be restricted to the section of metals which are characterized 

 " as incapable of absorbing oxygen or decomposing water at 

 any temperature, and whose oxides are reducible below a red 

 heat." The foregoing experiments further demonstrate that 

 the oxidizable metals, when so minutely divided as to be in 

 great measure freed from the control of cohesive attraction, 

 and at liberty to obey their natural affinities, do not at any 

 temperature determine the direct union of hydrogen with free 

 oxygen; their own more energetic affinity for oxygen pre- 

 dominating over the weaker affinity of hydrogen for oxygen, 

 and inducing the oxidation of metal in preference to the for- 

 mation of water. When raised to a low red heat, in contact 

 with hydrogen, and with access of air, the oxides of these 

 metals have been further shown to cause the combustion of 

 hydrogen, by yielding their oxygen to that element, and in- 

 stantly recombining with fresh atmospheric oxygen. The 

 continued incandescence thus exhibited, though apparently 

 identical with that of platina, has been traced to a series of 

 alternate reductions and reoxidations. 



When in a state of more compact aggregation, these metals, 

 it is true, effectuate gaseous combination, but only at a tem- 

 perature approaching that of boiling mercury*. In this state 



* It may be doubted whether, even in the compact state, the oxidizable 

 metals permit the union of hydrogen with oxygen, until their own surfaces 

 have combined with oxygen. For the metallic foil and turnings, which 

 were pure and bright when introduced into the mixed gases, were found 

 to be tarnished and covered with a film of oxide when withdrawn, after 

 having induced gaseous union. 



