THE GROWTH OF OUR KNOWLEDGE OF HELIUM. 255 



Kirchhoff's scale], the only one of my list, by the way, 

 which is not given on Mr. Lockyer's. This line, which was 

 conspicuous at the Eclipse of 1869, seems to be always 

 present in the spectrum of the chromosphere. . . . It has no 

 corresponding dark line in the ordinary solar spectrum, and 

 not improbably may be due to the same substance that 

 produces D 3 ." 



This same line was noted also by Lorenzoni and named 

 f. Another line at 4026 was added later by Professor 

 Young. 



Fig. 7. — Tacchini's observations of two slight solar disturbances 

 showing the height to which the layers of the different gases 

 extend. Magnesium vapour is highest of all, and is furthest 

 extended ; next comes a gas of still unknown origin, indicated 

 by a line at 1474 of Kirchhoff's scale and so on. 



Then with regard to solar disturbances. Let me refer 

 in detail to a diagram indicating some results arrived at by 

 the Italian observers. We are dealing with the spectro- 

 scopic record of two slight disturbances in a particular part 

 of the sun's atmosphere. The spectroscope tells us that in 

 that region there was a quantity of the vapour of magnesium 

 which is collected in that place. Then we find that another 

 substance, about which we again know nothing whatever, 

 is also visible in that region, and then we get the further 

 fact that in those particular disturbances we get four other 

 spectral lines indicated as being disturbed, and of those four 

 lines we only know about one. 



