252 Dr Wilson on the Decomposition of Water by Platinum 



steam by the electric spark, which is well known to have the 

 power of combining hydrogen and oxygen into water. A similar 

 experiment was made in perhaps a still more instructive form in 

 the latter part of last century by Bcccaria,* Pearson, and Van 

 Troostvvyk, and more recently by Wollaston,-}- in his well-known 

 decomposition of water by guarded poles. In certain of these trials, 

 it was found that Leyden jar discharges sent through water, de- 

 composed it till the accumulation of permanent gas left the wires 

 bare ; after which, the first spark that passed recombined the 

 gases into water, which again covered the wire, when decomposi- 

 tion could anew be obtained. Here, to appearance, the same agent 

 acting with the same intensity, alternately decomposed and recom- 

 posed water. For argument's sake, let it be acknowledged that 

 the heat alone of the spark was the cause of the chemical change. 

 Nevertheless, it may be questioned, whether it acted with equal in- 

 tensity in both cases. The electric spark must be conceived, accord- 

 ing to the results already given, to be at first at a high white heat, 

 and whilst retaining this temperature, 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, 

 acquires a power of uniting hydrogen and oxygen. Although, there- 

 fore, the spark is always /wrmVAec/ of the same intensity, its action 

 may change, and even be reversed, as its intensity diminishes. More- 

 over, 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. Con- 

 tiguous 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 com- 

 bination will begin, and ultimately extend, through the whole mass 

 of gas. 



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

 ture of hydrogen and oxygen, it causes their combination. Here we 

 may suppose that union occurs as soon as the temperature of the 

 metal rises to 660° F., and before it acquires a white heat. Or if 

 we were to arranfje 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 combination 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 pro- 

 pagated through a mixture of hydrogen and oxygen by a series of 



* Lettere dell* Elettricismo, quoted in Lardner's Electricity, vol. i., p. 78. 

 t Faraday's Electrical Kesearches, series 3, paragraph 328, 



