338 Prof. Tyndall on Luminous and Obscure Radiation. 



37. The rendering of metals incandescent by obscure rays 

 has not yet been accomplished. This is a question on which 

 Dr. Akin has been engaged for some years, and it is not my 

 intention to publish anything relating to it until the very pro- 

 mising arrangements which he has devised have had a sufficient 

 trial. 



38. Melloni's experiments led him to conclude that rock- 

 salt transmits obscure and luminous rays equally well, and that 

 a solution of alum of moderate thickness entirely intercepts the 

 invisible rays, while it allows all the luminous ones to pass. Hence 

 the difference between the transmissions of rock-salt and alum 

 ought to give the obscure radiation. In this way Melloni found 

 that 10 per cent, only of the radiation from an oil-flame consists 

 of luminous rays. The method above employed proves that 

 the proportion of luminous heat to obscure, in the case of an 

 oil-flame, is probably not more than one-third of what Melloni 

 made it. 



39. In fact this distinguished man clearly saw the possible 

 inaccuracy of the conclusion that none but luminous rays are 

 transmitted by alum ; and the following experiments justify the 

 clauses of limitation which he attached to his conclusion : — 



The solution of iodine was placed in front of the electric lamp, 

 the luminous rays being thereby intercepted. Behind the rock- 

 salt cell containing the opake solution was placed a glass cell, 

 empty in the first instance. The deflection produced by the 

 obscure rays which passed through both produced a deflec- 

 tion of 



80°. 



The glass cell was now filled with a concentrated solution of alum ; 

 the deflection produced by the obscure rays passing through both 

 solutions was 



50°. 



Calculating from the values of these deflections, it was found that 

 of the obscure heat emergent from the solution of iodine, and from 

 the side of the glass cell, 20 per cent, was transmitted by the alum. 



40. A point of very considerable importance forces itself upon 

 our attention here — namely the vast practical difference which may 

 exist between the two phrases, " obscure rays," and "rays from 

 an obscure source." Many writers seem to regard these phrases 

 as equivalent to each other, and are thus led into grave errors. 

 A stratum of alum solution Jgth of an inch in thickness is, ac- 

 cording to Melloni, entirely opake to the radiation from all bodies 

 heated under incandescence. In the foregoing experiments the 

 layer of alum solution traversed by the obscure rays of our lumi- 

 nous source was thirty times the thickness of the layer which 



