COMPOSITION OF A SUNBEAM. 49 



that it exists chiefly in part of the prismatic 

 spectrum where the red rays lie, and even 

 where there is no light visible. It is supposed 

 that this is accounted for by the rays of heat 

 being less bent out of their direction by the 

 prism than are the rays of light. We have 

 still got to show the existence of that curious 

 and interesting class of rays the Actinic. 

 This also may be readily done. A piece of 

 paper on which some solution of nitrate of 

 silver, or lunar caustic, as it is commonly called, 

 has been brushed, after it has been dried, 

 must be placed in the streak of coloured 

 light. After a little time it will be found 

 that the paper is blackened where the violet 

 and indigo colours shone, and even beyond 

 them, but less quickly, or to a very trifling 

 degree, where the other colours are placed. 

 The truth has been, that the nitrate of silver 

 lias been decomposed by the actinic, or chemical 

 rays which exist chiefly in the upper part of 

 this prismatic streak It is supposed that the 

 actinic rays are most bent out of their direction 

 by the prism, so that they appear at the highest 

 point of the spectrum. 



Thus we see that every ray pouring down 

 from the sun consists of light rays, of heat 

 rays, and of chemical or actinic rays. Need 

 it be said these have each a vastly important 



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