III.] THE PRESENT POSITION. 35 



Kirehhoff' s scale, which I stated to be coincident with a line of iron. 

 On June 26 I discovered another at 2003'4 of the same scale. 



The later researches on the spectrum of iron have shown that the 

 iron line which I observed in 1869 to be coincident with the bright 

 chromospheric line at 1474 on Kirehhoff' s scale, having a wave-length of 

 5316-79, is an enhanced line, agreeing absolutely with Young's latest 

 determination of the wave-length of the 1474 chromospheric line. 



Similarly the line at 2003'4 of Kirchhoff s scale, with a wave-length 

 of 4924, is also an enhanced line of iron. 



The first experiments were made to explain my own and the 

 Italian observations of the chromosphere which proved the presence of 

 only these two lines of iron in the part of the spectrum ordinarily 

 observed ; the ordinary spectrum of iron, in which 460 lines had been 

 mapped at that time, was entirely invisible. 



The anomalies were investigated in the experimental work with 

 sparks produced by quantity and intensity coils, with and without jars 

 in the circuit. The outcome of these experiments was to show that the 

 chromospheric representatives of iron were precisely the lines which 

 were brightened on passing from the arc to the spark, while the lines 

 widened in spots corresponded to a lower temperature. 



The next anomaly observed was that in a sun spot the iron line at 

 4924 often indicated no movement of the iron vapour, while the other 

 iron lines showed that it was moving with considerable velocity. 



It seemed perfectly clear then that in the sun " we were not 

 dealing with iron itself, but with primitive forms of matter contained 

 in iron which are capable of withstanding the high temperature of the 

 sun, after the iron observed as such has been broken up, as suggested 

 by Brodie."* 



On this view, the high temperature iron lines of the chromosphere 

 represent the vibrations of one set of molecules, while the lines which 

 are widened in spots correspond to other molecular vibrations. 

 Similarly, the idea of different molecular groupings provides a satis- 

 factory explanation of the varying rates of movement of iron vapour 

 indicated by adjacent lines, the lines being produced by absorption of 

 different molecules at different levels and at different temperatures. 



.-'' i 

 Magnesium. 



In 1879 I passed a spark through a flame charged with vapours of 

 different substances. In the case of magnesium the effect of the higher 

 temperature of the spark was very marked ; some of the flame lines 

 being abolished, while two new ones made their appearance, one of 



* Proc. Rot/. Soc., vol. xxxii, p. 234. 



D 2 



