1876.] Line- Spectra of Elementary Bodies. 353 



When a solid body such as a platinum wire, traversed by a voltaic cur- 

 rent, is heated to incandescence, we know that as the temperature increases, 

 not only does the radiation of each particular refrangibility absolutely 

 increase, but the proportion of the radiations of the different refrangibili- 

 ties is changed, the proportion of the higher to the lower increasing with 

 the temperature. It would be in accordance with analogy to suppose 

 that as a rule the same would take place in an incandescent surface, 

 though in this case the spectrum would be discontinuous instead of con- 

 tinuous. Thus if A, B, 0, D, E denote conspicuous bright lines, of in- 

 creasing refrangibility, in the spectrum of the vapour, it might very well 

 be that at a comparatively low temperature A should be the brightest and 

 the most persistent ; at a higher temperature, while all were brighter than 

 before, the relative brightness might be changed, and C might be the 

 brightest and the most persistent, and at a still higher temperature E. 

 If, now, the quantity of persistence were in each case reduced till all lines 

 but one disappeared, the outstanding line might be A at the lowest tem- 

 perature, C at the higher, E at the highest. If so, in case the vapour 

 showed its presence by absorption but not emission, it follows, from the 

 correspondence between absorption and emission, that at one temperature 

 the dark line which would be the most sensitive indication of the presence 

 of the substance would be A, at another C, at a third E. Hence, while I 

 regard the facts you mention as evidence of the high temperature of the 

 sun, I do not regard them as conclusive evidence of the dissociation of the 

 molecule of calcium. 



Tours sincerely, 



G. G. Stokes. 



5 Alexandra Eoad, 



Finohley Eoad, N.W., 

 March 5, 1876. 



Dear Professor Stokes, — I was not prepared for your suggestion, as 

 it was the abnormal and not the normal behaviour of Ca which led me 

 to investigate it. 



D is darker than any other of the Na lines, and H in the chromosphere 

 at the Ca level is red, while in the coronal atmosphere it is green ; i. e. 

 the least refrangible line is developed by increase of temperature, and 

 not the more refrangible one*. 



I am not the less grateful to you for your suggestion ; and so soon as 

 I can obtain the use of a more powerful coil I will go over the ground as 

 completely as I can. 



* [The point, however, is, Which is the most 'persistent line at the respective tempe- 

 ratures, i. e. the last to disappear when the quantity of substance present is continually 

 reduced ? and Mr. Lockyer himself has shown that the line which is the most con- 

 spicuous when there is substance enough present to show several lines is by no means 

 necessarily the most persistent. — G. G\ S.] 



