232 THE POPULAR SCIENCE MONTHLY. 



ciation tension of aqueous vapor at atmospheric pressure and at 2,800 

 C. is 0*5, that is to say one half of the vapor would exist as such, the 

 remaining half being found as a mechanical mixture of hydrogen and 

 oxygen ; but, with the pressure, the temperature- of dissociation rises 

 and falls, as the temperature of saturated steam rises and falls with its * 

 pressure. It is therefore conceivable that the solar photosphere may 

 be raised by combustion to a temperature exceeding 2,800 C, whereas 

 dissociation may be effected in space at- a lower temperature. This 

 temperature of 2,800 would be quite sufficient to account for the char- 

 acter and amount of solar radiation, if it is only borne in mind that 

 the luminous atmosphere may be a thousand miles in depth, and that 

 the flame of hydrogen and hydrocarbons, in the uppermost layers of 

 this zone, is transparent to the radiant energy produced in the layers 

 below, thus making the total radiation rather the sum of matter in 

 combustion than the effect of a very intensely heated surface. 



Sainte-Claire Deville's 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 important researches that 

 vapor 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 com- 

 pound substance 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 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 energy is such 

 as can be produced by an inferior source of heat. 



The point of dissociation of aqueous vapor and carbonic acid ad- 

 mits, however, of being determined by direct experiment. It engaged 

 my attention some years ago, but I have hesitated to publish the quali- 

 tative results I then obtained, in the hope of attaining to quantitative 

 proofs. 



These experiments consisted in the employment of glass tubes fur- 

 nished with platinum electrodes, and filled with aqueous vapor or with 

 carbonic acid in the usual manner, the latter being furnished with 

 caustic soda to regulate the vapor-pressure by heating. Upon immers- 

 ing one end of the tube charged with aqueous vapor in a refrigerating 

 mixture of ice and chloride of calcium, its temperature at that end was 

 reduced to 32 C., corresponding to a vapor-pressure, according to 

 Regnault, of l 8 1 u6 of an atmosphere. When so cooled no slow elec- 

 tric discharge took place on connecting the two electrodes with a small 



* See " Proceeding, 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. 



