RADIANT HEAT. 327 



The same lie considers established in regard to the blue end of the 

 spectrum and of the deoxidizing rays. He thinks it extremely probable 

 that the heating and illuminating rays are different rays, but they have 

 never yet been found in a state of complete separation. 



2d. Until it is proved, therefore, or rendered probable, that the 

 same intensity of light of different colors, as it proceeds directly from 

 tJie sun, is accompanied with different degrees of heat, we must 

 assume it as true that the heating power is proportional to the illumi- 

 nating power of the different rays of solar light. 



3d. It appears from Dr. Seebeck's experiments on the water 

 spectrum, that this relation holds generally in it, as he found the 

 maximum of heat to be in the yellow rays, or coincident with the maxi- 

 mum of light. Hence Sir D. Brewster draws the important conclusion 

 that water has tlf same degree of transparency for the solar heating rays 

 that if has for light, which is the same as all colorless transparent media 

 have for light; that is, water absorbs equally all the different rays ol 

 solar heat in the same manner as it does all the different rays of solar 

 light. 



4th. It has been found by experiment that with prisms of crown 

 glass the maximum heating effect is in the middle of fin' red space. 

 Unfortunately the relation between the maximum heed in the water 

 spectrum and in the crown glass spectrum has not been ascertained. 

 If we suppose them equal, it appears that the crown glass must have 

 exercised a greater cdtsorptive action than the water upon the more 

 refrangible rags, and a less absorptive action upon the less refrangible 

 rays, in the same manner as is done by red glasses upon light. 



A prism of sulphuric acid gives the maximum ordinate of heat in 

 the orange space, or the fluid absorbs more of the red rays than crown 

 glass, ami less of the rays on the other side of the orange. 



In flint glass, where the maximum heat is at the very extremity of 

 the spectrum, scarcely any of the red rays are absorbed, while great 

 proportions of all the others are. 



Dr. Turner (Chem., p. 84, 3d edit.) says that it is difficult to account 

 for Seebeck's results, without supposing that different media differ in 

 their power of refracting caloric, (t. e., the heating rays of the sun.) 



Sir D. Brewster considers that the true explanation is that which 

 the above principles afford, viz: that colorless transparent bodies, in 

 acting upon the solar heat, exercise the same sort of absorptive action 

 upon it that colored transparent bodies do upon light, the maximum 

 ordinate shifting its position with the nature of the body. Colored 

 media give sometimes two or more maxima of light, with large spaces 

 and small lines entirely defective of light, in consequence of the ab- 

 sorption being total at those places. 



In like manner he is persuaded it will be found that there are de- 

 fective spaces and lines in the spectrum of solar heat; these he thinks 

 may possibly be detected by using as thermometers the minute natural 

 cavities in topaz, &c, filled with fluid or vapor, and not more than 

 0.001 inch in magnitude. 



5th. These views are exactly accordant with the results of Sir W. 

 Herschel above stated. 



