HYDROGEN. 259 



Several circumstances affect the combination of hydrogen 

 with oxygen, which are important. These gases may be mixed 

 together in a glass vessel, and preserved for any length of time 

 without combining. But combination is instantly determined 

 by flame, by passing the electric spark through the mixture, or 

 even by introducing into it a glass rod, not more than just 

 visibly red-hot. Hydrogen, indeed, is one of the more easily 

 inflammable gases. If the mixed gases be heated in a vessel 

 containing a quantity of pulverized glass, or any sharp powder, 

 they begin to unite in contact with the foreign body in a gradual 

 manner without explosion, at a temperature not exceeding 

 660. The presence of metals disposes them to unite at a still 

 lower temperature; and of the metals, those which have 

 no disposition of themselves to oxidate, such as gold and 

 platinum, occasion this slow combustion at the lowest tem- 

 perature. In 1824, Dobereiner made the remarkable dis- 

 covery that newly prepared spongy platinum has an action 

 upon hydrogen independently of its temperature, and quickly 

 becomes red-hot when a jet of this gas is thrown upon it in air, 

 combination of the gases being effected by their contact with 

 the metal. In consequence of this ignition of the platinum the 

 hydrogen itself is soon inflamed, as it issues from the jet. An 

 instrument depending upon this actkm' of platinum has been 

 constructed for producing an instantaneous light. More lately 

 Mr. Faraday observed that the divided state of the platinum, 

 although favourable, is not essential to this action ; and that a 

 plate of that metal, if its surface be scrupulously clean, will 

 cause a combination of the gases, accompanied with the same 

 phenomena, as the spongy platinum. This action of platinum 

 is manifested at temperatures considerably below the freezing 

 point of water, and in an explosive mixture largely diluted with 

 air or hydrogen. Spongy platinum, made into pellets with a little 

 pipe-clay, and dried, when introduced into mixtures of oxygen and 

 hydrogen, will be found to cause a gradual and silent combina- 

 tion of the gases, in whatever proportions they are mingled, 

 which will not cease till one of them is completely exhausted. 

 The theory of this effect of platinum is very obscure. It be- 

 longs to a class of actions depending upon surface, not confined 

 to that metal, and by which other combustible vaporous bodies 

 are affected besides hydrogen (page 196.) 



The flame of hydrogen, although so slightly luminous, is in- 



s 2 



