186 Dr. Wilson on the Decomposition of Water by Platinum 



anew be obtained. Here, to appearance, the same agent act- 

 ing with the same intensity, alternately decomposed and re- 

 composed water. For argument's sake, let it be acknow- 

 ledged that the heat alone of the spark was the cause of che- 

 mical change. Nevertheless it may be questioned, whether 

 it acted with equal intensity in both cases. The electric spark 

 must be conceived, according to the results already given, to 

 be at first at a high white heat, and whilst retaining this tem- 

 perature we may believe it to possess a power of disuniting 

 the elements of water, and of preventing their union. But as 

 soon as the spark falls to the temperature of 660° F., it loses 

 its power of decomposing water, and, on the other hand, ac- 

 quires a power of uniting hydrogen and oxygen. Although 

 therefore the spark is always furnished of the same intensity, 

 its action may change, and even be reversed, as its intensity 

 diminishes. Moreover, even when the spark is white-hot, it 

 is only the amount of matter directly in its track that will be 

 raised to a white heat. Contiguous portions will have their 

 temperature much lower, so that in the case of hydrogen and 

 oxygen, at some little distance from the route of the spark, 

 the temperature will be 660° F., and there combination will 

 begin, and ultimately extend through the whole mass of gas. 

 In like manner, when a platinum wire is made white-hot in a 

 mixture of hydrogen and oxygen, it causes their combination. 

 Here we may suppose that union occurs as soon as the tem- 

 perature of the metal I'ises to 660" F., and before it acquires 

 a white heat. Or if we were to arrange matters so that the 

 wire should be made white-hot in a vacuum and hydrogen 

 and oxygen afterwards admitted to it, still union of the gases 

 should happen ; for although the wire might prevent com- 

 bination immediately around itself, at no great distance where 

 the temperature was below 700° F. it would compel union. 

 In all such experiments the combining effect of heat will be 

 much more manifest than its decomposing power ; not that 

 perhaps the former is in reality greater than the latter, but 

 because flame is propagated through a mixture of hydrogen 

 and oxygen by a series of combustions. The hot wire or the 

 electric spark kindles only the portions of gas immediately 

 adjacent to it, but the combustion of those sets fire to the 

 molecules contiguous to them, and these in their turn to their 

 neighbours, till all are made to burn. Thus the flame travels 

 after the original cause of combustion has ceased to operate 

 directly, and the momentary action of a small spark, or the 

 transient heat of a red-hot capillary wire may suffice to fire 

 an infinitely large mass of hydrogen and oxygen. There is 

 no provision for a similar propagation of decomposition 

 through water or steam when either is made white-hot j the 



