66 



On a Gas showing the Spectrum of Helium. [Apr. 2f>, 



whether magnesium-poles would free the argon from all traces of 

 nitrogen. This it did ; but hydrogen was evolved from the magne- 

 sium, so that its spectrum was distinctly visible. Moreover, 

 magnesium, usually contains sodium, and the D line was also visible, 

 though faintly, in the argon-tube. The gas from cleveite also 

 showed hydrogen lines dimly, probably through not having been 

 filled with completely dried gas. 



On comparing the two spectra, I noticed at once that while the 

 hydrogen and argon lines in both tubes accurately coincided, a 

 brilliant line in the yellow, in the cleveite gas, was nearly but not 

 quite coincident with the sodium line D of the argon-tube. 



Mr. Crookes was so kind as to measure the wave-length of this 

 remarkably brilliant yellow line. It is 587 '49 millionths of a 

 millimetre, and is exactly coincident with the line D 3 in the solar 

 chromosphere, attributed to the solar element which has been named 

 helium. 



Mr. Crookes has kindly consented to make accurate measurements 

 of the position of the lines in this spectrum, which he will publish, 

 and I have placed at his disposal tubes containing the gas. I shall 

 therefore here give only a general account of the appearance of the 

 spectrum. 



While the light emitted from a Pfliicker's tube charged with argon 

 is bright crimson, when a strong current is passed through it, the 

 light from the helium-tube is brilliant golden yellow. With a feeble 

 current the argon-tube shows a blue-violet light, the helium-tube a 

 steely blue, and the yellow line is barely visible in the spectroscope. 

 It appears to require a high temperature therefore to cause it to 

 appear with full brilliancy, and it may be supposed to be part of the 

 high-temperature spectrum of helium. 



The following table gives a qualitative comparison of the spectra 

 in the argon* and in the helium-tubes. 



* The tube then used was the one with which Mr. Crookes's measurements of the 

 argon spectrum, were made. It contains absolutely pure atmospheric argon. 



