THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MARCH 1883. 



XXII. The Selective Absorption of Solar Energy. 

 By S. P. Langley*. 



[Plate III] 



Introduction. 



IN 1800 Sir William Herschel published his remarkable 

 investigations, in the Philosophical Transactions, on 

 obscure heat, in which he reached the conclusion that heat is 

 an entity distinct from light. 



This view was modified by subsequent writers into the 

 statement that each ray has three qualities — heat, light, and 

 chemical action; while at the present time many physicists 

 have reached the further conception that heat, light, and che- 

 mical action are not so much qualities inherent in any ray, as 

 modes of the manifestation of one common energy — or, to 

 state this view in the broadest manner, that the same sethereal 

 wave will give us either heat, light, or chemical action, accord- 

 ing to the absorbent nature of the substance which receives it. 



These last opinions, however, cannot be said even yet to be 

 universally accepted by physicists. 



Dr. J. W. Draper long since pointed out that the maximum 

 of heat did not necessarily lie in every case below the red 

 (though it does so in the prismatic spectrum) , and that in a 

 normal spectrum it would be in the orange. These conclu- 



* Communicated by the Author, to whom we are also indebted for the 

 cliches of the illustrations. 



Phil. Mag. S. 5. Yol. 15. No. 93. March 1883. M 



