SECT, xxv.] SOLAR SPECTRUM. ^39 



porated the spirit first on those parts of the paper where it 

 fell with greatest intensity, thereby restoring their white 

 colour, and thus he discovered that the caloric is not dis- 

 tributed uniformly, but in spots of greater or less intensity 

 a circumstance probably owing to the absorbing action of 

 the atmospheres of the sun and earth. " The effect of the 

 former," says Sir John, " is beyond our control, unless we 

 could carry our experiments to such a point of delicacy, as 

 to operate separately on rays emanating from the centre and 

 borders of the sun's disc ; that of the earth's, though it 

 cannot be eliminated any more than in the case of the sun's, 

 may yet be varied to a considerable extent by experiments 

 made at great elevations, and under a vertical sun, and com- 

 pared with others where the sun is more oblique, the situa- 

 tion lower, and the atmospheric pressure of a temporarily 

 high amount. Should it be found that this cause is in 

 reality concerned in the production of the spots, we should 

 see reason to believe that a large portion of solar heat 

 never reaches the earth's surface, and that what is incident 

 on the summits of lofty mountains differs not only in quan- 

 tity, but also in quality, from what the plains receive." 



Thus the solar spectrum is proved to consist of five super- 

 posed spectra, only three of which are visible the red, 

 yellow, and blue ; each of the five varies in refrangibility 

 and intensity throughout the whole extent, the visible part 

 being overlapped at one extremity by the chemical, and at 

 the other by the calorific rays ; but the two latter exceed 

 the visible part so much, that the linear dimensions of the 

 three, the luminous, calorific, and photographic, are in the 

 proportion of the numbers 25, 42, 10, and 55-10, so that the 

 whole solar spectrum is more than twice as long as its 

 visible part. 



That the heat-producing rays exist independently of light, 

 is a matter of constant experience in the abundant emission 

 of them from boiling water. Yet there is every reason to 

 believe that both the calorific and chemical rays are modi- 



