and the Black Oxide of Iron at a white heat. 185 



it is decomposed into mercury and oxygen. In like manner, 

 if hydrogen and oxygen be raised together to the tempera- 

 ture of 660° F.*, they unite and form water. If the resulting 

 Mater be raised to a white heat, it is resolved into hydrogen 

 and oxygen. Both metals (?) present the same phaenomena. 

 At one temperature (nearly the same in both cases) combina- 

 tion with oxygen occurs ; at a higher temperature, decompo- 

 sition of the oxide happens. Many other examples might be 

 given in illustration of the same fact. Such cases, however, 

 do not seem to warrant a conclusion as to heat exhibiting 

 anything like a polarity of force, by which I understand the 

 manifestation in opposite directions of opposite powers of 

 equal intensity. At all events, if the opposite effects of dif- 

 ferent intensities of the same agent be considered equivalent 

 to a polarity of action, it is difficult to see what force may not 

 be called a polar one. The decomposing and combining 

 power of heat of different intensities, seems exactly compara- 

 ble to the opposite effects of different intensities of mechanical 

 impulse. 



If two pieces of smooth glass are laid together and struck 

 gently or compressed slightly, they unite or cohere. If the 

 united pieces are thereafter exposed to a sharp blow or to 

 great compression, the union is dissolved, or they are shat- 

 tered to fragments. Here the same force effects mechanical 

 synthesis and mechanical analysis. But in these contrasted 

 actions, as seems to be the case also in Mr. Grove's experi- 

 ments, the results are occasioned by a difference in degree of 

 intensity of the same power, not as in the opposite effects of 

 a polarizing force like electricity, by a difference in the kind 

 of power which appears, whatever be its intensity. There is 

 one form, indeed, of Mr. Grove's experiment which at first 

 sight does not appear to admit of the explanation proposed 

 in reference to the other trials — I allude to the decomposition 

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

 the poM^er of combining hydrogen and oxygen into water. 

 A similar experiment w^as made in perhaps a still more in- 

 structive form in the latter part of last century by Beccariaf, 

 Pearson and Van Troostwyk, and more recently by WoUas- 

 ton J, in his well-known decompositions of water with guarded 

 poles. In certain of these trials it was found that Leyden 

 jar discharges sent through water, decomposed it till the ac- 

 cumulation 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 decomposition could 



* Graham's Elements, 1st edit. p. 259. 



t Lettere dell' Electrecismo, quoted in Lardner's Electricity, vol. i.p. 78. 



j Faraday's Electrical Researches, series 3, paragraph 328. 



