S//? WILLIAM SIEMENS, F.R.S. 429 



raised by combustion to a temperature exceeding 2800 C., whereas 

 dissociation may be effected in space at comparatively low tempera- 

 tures. 



These investigations had reference only to heats measured by 

 means of pyrometers, but do not extend to the effects of radiant 

 lint. Dr. Tyndall has shown by his exhaustive researches that 

 vapour of water and other gaseous compounds intercept radiant 

 heat in a most remarkable degree, and there is other evidence to 

 show that radiant energy from a source of high intensity possesses 

 a dissociating power far surpassing the measurable temperature to 

 which the compound substance under its influence is raised. Thus 

 carbonic anhydride and water are dissociated in the leaf cells of 

 plants, under the influence of the direct solar ray at ordinary 

 summer temperature, and experiments in which I have been en- 

 gaged for nearly three years * go to prove that this dissociating 

 action is obtained also under the radiant influence of the electric 

 arc, although it is scarcely perceptible if the source of radiant 

 energy is such as can be produced by the combustion of oil or 

 gas. 



The point of dissociation of aqueous vapour and carbonic 

 anhydride admits, however, of being determined by direct experi- 

 ment. It engaged my attention some years ago, but I have 

 hesitated to publish the qualitative results T then obtained, in the 

 hope of attaining to quantitative proofs. 



These experiments consisted in the employment of glass tubes, 

 furnished with platinum electrodes, and filled with aqueous vapour 

 or with carbonic anhydride in the usual manner, the latter being 

 furnished with caustic soda to regulate the vapour pressure by 

 heating. Upon immersing one end of the tube charged with 

 aqueous vapour in a refrigerating mixture of ice and chloride of 

 calcium, its temperature at that end was reduced to 82 C., 

 corresponding to a vapour pressure, according to Regnault, of 

 TsVo-th of an atmosphere. When so cooled no slow electric discharge 

 took place on connecting the two electrodes with a small induction 

 coil. I then exposed the end of the tube projecting out of the 

 freezing mixture, backed by white paper, to solar radiation (on a 



* See Proceedings of the Royal Society, Vol. XXX., p. 208, and Paper read 

 before Section A., British Association, and printed in full in the Report for 

 1881, Part I., p. 474, p. 252, ante. 



