and the Btack Oxide of Iron at a white heat. 251 



posite or dissimilar chemical effects in the same circumstances. He 

 is reported in the Athenaeum (Sept. 19, 1846, p. 966,) to have 

 '* announced his discovery that all the processess by which water may 

 be formed are capable of decomposing water," (p. 966). If, by 

 this statement, be simply meant, that heat combines oxygen and 

 hydrogen into water, and decomposes water into these gases, it will 

 be admitted to be a just conclusion ; but it may be questioned, I 

 think, whether Mr Grove's experiments add anything to our know- 

 ledge of the power of heat to effect chemical changes, except in so 

 far as they supply an additional very remarkable example of its two- 

 fold analytical and synthetical agency, which has been so long recog- 

 nised. Hydrogen, which as a gas, is probably the vapour of a very « 

 volatile metal, may be compared with mercury, also a volatile sub- 

 stance. If mercury and oxygen be heated together to the tempera- 

 ture of 662° F., they combine and form the red oxide of the metal. 

 If the resulting oxide be raised to a low red heat, it is decomposed 

 into mercury and oxygen. In like manner, if hydrogen and oxy- 

 gen be raised together to the temperature of 660° F.,* they unite 

 and form water. If the resulting water be raised to a white heat, 

 it is resolved into hydrogen and oxygen. Both metals (?) present 

 the same phenomena. At one temperature (nearly the same in both 

 cases) combination with oxygen occurs ; at a higher temperature, 

 decomposition 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 op- 

 posite directions of opposite powers of equal intensity. At all events, 

 if the opposite effects of different intensities of the same agent be con- 

 sidered equivalent to a polarity of action, it is difficult to see what 

 force may not be called a polar one. The decomposino- and combin- 

 ing power of heat of different intensities, seems exactly comparable 

 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 shattered 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 experiments, the results are occasioned by a diff'erence 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 



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



