xiv.] YOUNG'S WORK. 185 



Especially noticeable were two lines at w.l. 4923*1, 5017'6, 

 given in my list, which were seen with great frequency till 

 1873, when they disappeared, and two new lines, about which 

 nothing is even yet known, came in their place. 1 



About this time an American astronomer, Professor Young, 

 made an important series of observations during an expedition 

 to Sherman, a point 8,000 feet high in the Eocky Moun- 

 tains. He saw an immense number of bright lines in the 

 prominences, but a study of them only served to show our 

 ignorance. Thus the longest iron line in the region between 

 F and b was only seen once, while a much fainter iron line at 

 49231 was seen forty times. Another iron line at 491 8 '2 was 

 seen twenty times, while an equally strong line at 491 9'8 was not 

 seen at all. In my observations I only saw three iron lines out 

 of 460, and this, as I have shown, was confirmed by Vogel and 

 Tacchini. Young, with much greater advantages as regards 

 climate, was able to see 110 lines. He also saw the H and K 

 calcium lines with a frequency of seventy-five and fifty re- 

 spectively, while the much stronger blue calcium line at w.l. 

 4226*3 was seen only three times. 2 In the reduction of these 

 observations he pointed out the fact that the lines which he 

 had seen most frequently were lines common to two or more 

 elements. He writes : 3 



" Two explanations suggest themselves. The first, which seems 

 rather the more probable is, that the metals operated upon by the 

 observer who mapped their spectra were not absolutely pure 

 either the iron contained traces of calcium and titanium, or vice 

 versd. If this supposition is excluded, then we seem to be driven to 

 the conclusion that there is some such similarity between the mole- 

 cules of the different metals as renders them susceptible of cer- 

 tain synchronous periods of vibrations a resemblance, as regards 

 the manner in which the molecules are built up out of the 



1 Tacchini, Memoire della Societa degli spettroscopisti Italiani, vol. i. p. 89 ; 

 vol. ii. pp. 55, 59, 60, 62, 95 ; vol. iii. p. 95 ; vol. iv. pp. 81 et seq. 

 " Young, United States Coast Survey Report, 1872. 

 3 Nature, vol. vii. p. 17 et seq. 



