240 Dr. Miiller on the Thermal Effects 



separate portions of the spectrum are obtained, which are shown 

 in the fourth column of the foregoing table. 



Accordingly the heating power 

 Of all violet, indigo, and blue rays of the spectrum would be 13 



Of all green rays of the spectrum 13 



Of all yellow and orange rays of the spectrum (74—40; . 34 



Concerning the heating power of all the red rays of the 

 spectrum, we carmot consider the number 40, corresponding to 

 the red solution, as expressing this power immediately, because 

 we ai-e not justified from our experiments in assuming that the 

 red solution allows only red, and not a certain amount of dark 

 thermal rays, to pass through. 



A red liquid, which transmits only red rays exclusively of the 

 dark thermal rays, is probably as hypothetical as a yellow liquid 

 which only allows the yellow and no red rays to pass through. 



According to the experiments of Franz (Pogg. Ann. vol. ci. 

 pp. 57, 58), we may assume that fi'om 43 to 50 per cent, of all 

 rays passing through the red solution are dark thermal rays. 

 Hence, for the thermal power of all the red i-ays of the solar 

 spectrum, we should have left the value 20 to 30. 



From these experiments, it follows that the thermal power of 

 the less refrangible rays of the solar spectrum, namely the red, 

 orange, and yellow, is much greater than that of the green, blue, 

 indigo, and violet. The numerical values given above, taken in 

 conjunction with the extension of the separate colours in the 

 spectrum, would lead to the construction of a curve of the inten- 

 sity of the thermal force in the solar spectrum. We may, how- 

 ever, put this on one side ; for the results obtained could not fail 

 to be less exact than the one derived from a direct examination 

 of the solar spectrum in its thermal relations. 



Experiments on the distribution of heat in the spectra of a glass 

 and of a rock-salt prism. 

 After completing the series of experiments described above, I 

 proceeded to direct experiments on the distribution of heat in the 

 solar spectrum, having obtained a lineal thermo-battery. A 

 solar spectrum in which none of the thermal rays are absorbed 

 can only be obtained, as is well known, by means of a prism of 

 rock-salt. Before employing such a one, which I had got from 

 J. V. Albert Sohn in Frankfort, I thought it well to experiment 

 at first with a glass prism, in order to find out the best arrange- 

 ment of the experiments, and to acquire the necessary practice in 

 performing them, before removing the rock-salt prism from the 

 glass case in which it was cemented. 



After some preliminary experiments, the following arrange- 

 ment of the apparatus was adopted : — 



