22 



Mr. G. J. Stoney on the Physical 



[Recess,, 



spicuous as dark rays. These same rays, since they are due to feeble 

 atomic motions, will, in the iron spectrum produced by artificial means, 

 appear the faintest. Now in all regions of the iron spectrum artificially 

 produced, rays present themselves of every possible degree of intensity ; 

 whereas of those observed by Kirchhoff in the solar spectrum, the fluc- 

 tuation of intensity in any one region of the spectrum seldom exceeds one 

 degree of his numerical scale, and but once exceeds two degrees. This is 

 conclusive evidence that iron is so very abundant in the solar atmosphere as 

 to be opaque for the feeblest of these rays before a depth is reached which 

 is very much hotter than the outer surface of the iron atmosphere. It 

 also shows that the gradation of brightness in the iron lines from the more 

 to the less refrangible parts of the spectrum is not due to the less refran- 

 gible lines coming from profound depths, and being on this account 

 brighter. But the cause is sufficiently obvious. If a body of such a kind 

 that it emits the maximum light corresponding to its temperature, be 

 gradually heated, it will first begin to glow with scarlet, orange, yellow, and 

 green rays ; and according as its temperature rises its spectrum will ex- 

 pand in both directions towards the extreme red, and still more towards 

 the violet. If, then, a body heated in a furnace be compared with one 

 at a much higher temperature, the spectrum of the former will every- 

 where be fainter than that of the latter, but not equally so. It may have 

 a considerable brightness in the red and orange rays, and show sensible 

 light in the green, and at the same time appear in the comparison abso- 

 lutely black at higher refrangibilities. And the same general appearance* 

 would doubtless be found if the maximum spectrum of any one tempera- 

 ture were compared with the maximum spectrum of a higher tempera- 

 ture f. Now the upper layer of the iron atmosphere, from which comes 

 all the light that reaches us in the iron lines of the sun's spectrum, is at a 

 vastly lower temperature than the photosphere, but not so cool as to be of 

 insensible brightness through the whole range of the spectrum. It begins 

 to glow sensibly in the green, even in comparison with the intense light of 

 the sun, and renders the iron lines of the green short of absolute blackness. 

 And this effect goes on increasing until it reaches its climax in the orange 

 and red. 



31. As molecules of calcium vapour are of a mass less than that of iron 

 molecules, in the ratio of 40 to 56, calcium vapour must reach a far cooler 



* See § 52. 



t It is natural to suppose that this steady increase of intensity with the temperature 

 which pervades the whole range of the visible spectrum, should extend beyond it ; and 

 we are assured of it by the phenomena of calorescence. Dr. Tyndall succeeded in 

 heating a body so as to be visible by the concentration upon it of rays beyond the red. 

 This would have been impossible, — it would have been at variance with the principles of 

 the exchange of heat, if'the rays which were brought together were of an intensity that 

 could be emitted by a non-luminous source. Hence the source from which they came 

 (which was in fact a far hotter body whose luminous rays had been intercepted) was 

 able to send forth invisible rays more intense than any non-luminous body could emit. 



