Natural Classification of Elementary Substances. 467 



Runge further observed that the less refrangible of the two 

 lines was much weaker than the other, and he found the 

 difference in the wave-lengths of the lines to be 0*323 tenth 

 metres. 



The doubleness of the yellow line of the cleveite gas led 

 directly to a closer observation of the chromospheric line by 

 Huggins"*, Lockyer, and Professor Hale of Chicago f. Each 

 of these observers has found that the solar helium line is also 

 double. Mr. T. Thorp of Whitefield, the inventor of several 

 valuable spectroscopic appliances to the telescope, has also, 

 by means of a Rowland grating of 14,438 lines to the inch, 

 and a fourth-order spectrum, divided D 3 . Through the 

 kindness of Mr. Thorp, 1 have been able to confirm the obser- 

 vations which have, up to the present time, been made on the 

 doubleness of the yellow line of solar helium. 



Not only is the doubleness of the chromospheric line 

 established, but its components are of unequal width, and the 

 weaker line is on the least refrangible side of the spectrum as 

 in the gas from cleveite. Prof. Hale has determined the 

 difference in the wave-lengths of the components of D 3 to 

 be 0*357. Moreover, Lockyer has observed that five other 

 prominent lines of the new gas coincide with the chromo- 

 spheric lines 7066, 6678, 5016, 4922, and 4472 J. The only 

 question now open for discussion as to the identity of solar 

 and terrestrial helium is the difference in the wave-lengths of 

 the double line as determined by the several observers. 

 Crookes, as we have seen, pronounced the yellow line of 

 terrestrial helium to be single. Ramsay subsequently ob- 

 served the line double, and estimated the distance between 

 them to be g^ part of that between the sodium lines § = 

 0*120 tenth metres. Runge and Paschen observed a differ- 

 ence of 0*323 between the components of the yellow line, 

 while Prof. Hale, as I have said, makes the difference 

 between the same components of solar helium 0*357. 



In none of these observations of the characteristic yellow 

 line of terrestrial and solar helium would any account appear 

 to have been taken of the influence of pressure and diffusion 

 with other gases, in varying the width of spectral lines, espe- 

 cially on the more refrangible side of the spectrum. I have 

 already shown in my paper on the spectrum of thallium 

 (Proc. Roy. Soc. 1893) that the expansiveness and bright- 

 ness of the C line of hydrogen, at atmospheric pressure, 

 masked completely one of the two principal lines in the 



* Chemical News, July 19, p. 27. 



t Nature, August 1, p. 327. 



X Proc. Roy. Soc. June 13, 1895. 



§ Paper read before the Chemical Society, June 20, 1895. 



