On the Chemistry of the Hottest Stars. 



189 



4. In sun spots we deal with one set of iron lines, in the chromo- 

 sphere with another. 



5. At the maximum sun spot period the lines widened in spot 

 spectra are nearly all unknown ; at the minimum they are chiefly 

 due to iron and other familiar substances. 



6. The up-rush or down-rush of the so-called iron vapour in the 

 sun is not registered equally by ail the iron lines, as it should be on 

 the non-dissociation hypothesis. Thus, as I first observed in 1880,* 

 while motion is sometimes shown by the change of refrangibility of 

 some lines attributed to iron, other adjacent iron lines indicate a 

 state of absolute rest. 



Laboratory work without stint has been brought to bear, with a 

 view of attempting to explain the anomalies to which attention has 

 been directed. 



It is desirable to refer briefly to some of the work undertaken in 

 relation to the anomalies in question. 



1 . Experiments on calcium to determine cause of inversion of the 

 intensities of H and K. 



2. Experiments on magnesium to explain the inverted intensities 

 of the lines in the Fraunhofer spectrum. 



3. Experiments on iron to explain the Italian observations of the 

 chromosphere which proved the presence of only two lines of iron in 

 the part of the spectrum ordinarily observed. 



Some Special Experiments. 



Calcium. — In 1876 I produced evidence that the working 

 hypothesis that the molecular grouping of calcium which gives a 

 spectrum having its principal line at 4226"9 is nearly broken up in 

 the sun, and quite broken up in the spark, explained the facts, which 

 are that the low temperature line loses its importance and practically 

 disappears from the spectrum of the sun, in which H and K are by 

 far the strongest lines. t 



I summed up the facts regarding calcium as follows :J " We have 

 the blue line differentiated from H and K by its thinness in the solar 

 spectrum while they are thick, and by its thickness in the arc while 

 they are thin. We have it again differentiated from them by its 

 absence in solar storms in which they are almost universally seen, 

 and, finally, by its absence during eclipses, while the H and K lines 

 have been the brightest seen or photographed." 



I afterwards attempted to carry the matter further by photograph- 

 ing the spectra of sun spots. In all cases H and K lines were seen 



* ' Roy. Soc. Proc./ IS T o., 207, 1880. 

 f Ibid., vol. 24, p. 352. 



* Ibid., vol. 28, p. 171. 



