204 HERMANN VON HELMHOLTZ 



was now working at subjects with which du Bois was not 

 much concerned, this was replaced by an increasingly 

 intimate correspondence by word and letter with W. Thomson 

 (Lord Kelvin), in which they not only discussed the epoch- 

 making work they were themselves engaged upon, but com- 

 municated to one another the most important researches 

 and discoveries of other observers during the long period 

 of nearly fifty years. In this way Helmholtz was the first 

 to inform Lord Kelvin of KirchhofFs discovery of metals in 

 the solar atmosphere. Although the letter in question can 

 no longer be found by Lord Kelvin, he addressed the following 

 very interesting lines to the author on September 26, 1902 : 



' There should be several others between that date and 1856, 

 when I first had the great pleasure of making personal 

 acquaintance with Helmholtz in Kreutznach when he came 

 to see me, and in Bonn where I returned his visit. 



' There should be a letter of November or December, 1859, 

 telling me of Kirchhoffs discovery of metals in the solar 

 atmosphere by spectrum analysis. You may possibly find 

 my answer which I wrote immediately on receiving it, telling 

 him that as chanced two or three days before, I had, in 

 a lecture to my students in Glasgow University, told them 

 that I had learned from Stokes that the double dark line D 

 in the spectrum of sunlight proves that there is sodium vapour 

 in the sun's atmosphere, and that other metals might be found 

 there by the comparison of the Fraunhofer dark lines in 

 the solar spectrum with the dark lines produced in flames 

 by metals. I am sure I must also have told him that I had 

 been giving this doctrine regularly in my lectures for several 

 years. 



' I well remember that at that time I was making " Properties 

 of Matter" the subject of my Friday morning lecture. On 

 one Friday morning I had been telling my students that we 

 must expect the definite discovery of other metals in the sun 

 besides sodium by the comparison of Fraunhofer's solar dark 

 lines with artificial bright lines. The next Friday morning 

 I brought Helmholtz's letter with me into my lecture and read 

 it, by which they were told that the thing had actually been 

 done with splendid success by Kirchhoff.' 



After the publication of the Second Part of Physiological 



