394 



Dr. C. W. Siemens. 



[Mar. 2, 



as the temperature of saturated steam rises and falls with its pressure. 

 It is therefore conceivable that the temperature of the solar photo- 

 sphere may be raised by combustion to a temperature exceeding 

 2800° C, whereas dissociation may be effected in space at compara- 

 tively low temperatures. 



These investigations had reference only to heats measured by 

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

 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 engaged 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 experiment. It 

 engaged my attention some years ago, but I have hesitated to publish 

 the qualitative results I 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 fur- 

 nished 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 —32° C, corresponding to a vapour pres- 

 sure, according to Regnault, of -fsVo °^ an atmosphere. When so 

 cooled no slow electric discharge took place on connecting the two elec- 

 trodes 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 clear summer's day) for several hours, when 

 upon again connecting up to the inductorium, a discharge, apparently 

 that of a hydrogen vacuum, was obtained. This experiment being 

 repeated furnished unmistakable evidence, I thought, that aqueous 

 vapour had been dissociated by exposure to solar radiation. The C0 2 

 tubes gave, however, less reliable results. Not satisfied with these 

 qualitative results, I made arrangements to collect the permanent 



* See " Proc. Roy. Soc," vol. 30, p. 208, and paper read before Section A, British 

 Association, and printed in full in the Eeport for 1881, Part I, p. 474. 



