83Q Royal Society: — Mr. J. N. Lockyer on the 



March 3, 1876. 



Mx dear Lockyer, — You might perhaps like that I should put 

 on paper the substance of the remarks I made last night as to the 

 evidence of the dissociation of calcium. 



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

 taic current, is heated to incandescence, we know that as the tempe- 

 rature increases, not only does the radiation of each particular 

 refrangibility absolutely increase, but the proportion of the radia- 

 tions of the different refrangibilities 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 continuous. Thus 

 if A, B, C, D, E denote conspicuous bright lines, of increasing re- 

 frangibility, in the spectrum of the vapour, it might very w T ell 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 temperature, C at the higher, E at 

 the highest. If so, in case the vapour showed its presence by ab- 

 sorption but not emission, it follows, from the correspondence be- 

 tween 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 tem- 

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

 the dissociation of the molecule of calcium. 



Tours sincerely, 



G-. Gr. Stokes. 



5 Alexandra Koad, Finchley Eoad, N.W., 

 March 5, 1876. 



Deah Proeessoe, Stokes, — I was not prepared for your sug- 

 gestion, 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 chromo- 

 sphere 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. 



Are you quite sure that the molecular structure of the platinum 

 wire is constant while it behaves as you say it does ? 



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

 temperatures, 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 conspicuous when there is substance enough present to show several 

 lines is by no means necessarily the most persistent. — Gh Gr. S.] 



