IV U.!.l.\. M SIEMENS, l-.R.S. 305 



passing the measurable temperature to which the compound sub- 

 stance under its influence is raised. Thus carbonic acid and 

 water are dissociated in the leaf-cells of plants under the influence 

 of the direct solar ray at ordinary summer temperature, and experi- 

 iin ins 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 per- 

 ceptible if the energy is such as can be produced by an inferior 

 source of heat. 



The point of dissociation of aqueous vapour and carbonic acid 

 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 acid 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 eud was reduced to - 32 C., corresponding 

 to a vapour pressure, according to Regnault, of -oVoth 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 

 clear summer's day) for several hours, when upon again con- 

 necting up to the inductorium, a discharge, apparently that of a 

 hydrogen vacuum, was obtained. This experiment being repeated 

 furnished unmistakeable evidence, I thought, that aqueous vapour 

 had been dissociated by exposure to solar radiation. The carbonic 

 acid tubes gave, however, less unmistakeable effects. Not satisfied 

 with these qualitative results, I made arrangements to collect the 

 permanent gases so produced by means of a Sprengel pump, but 

 was prevented by lack of time from pursuing the inquiry, which I 

 propose, however, to resume shortly, being of opinion that, in- 



* See Proceedings, Royal Society, Vol. XXX., March 1, 1880 ; also a paper 

 read before Section A of the British Association, September 1, 1881, and ordered 

 to be printed in the Report. 



VOL. III. X 



