VIL. On the Spectra of Argon. 
By Wiui1aM Crookes, F.RS., &c. 
Received January 26,—Read January 31, 1895. 
[PLATE 3. | 
THrovGH the kindness of Lord RayLercH and Professor Ramsay I have been enabled 
to examine the spectrum of this gas in a very accurate spectroscope, and also to take 
photographs of its spectra in a spectrograph fitted with a complete quartz train. 
The results are both interesting and important, and entirely corroborate the con- 
clusions arrived at by the discoverers of argon. 
The results of my examination are given in a table of wave-lengths, which follows, 
and on a map of the lines accurately drawn to scale, accompanying this paper (Plate 3). 
The map is 40 feet long, and the probable error of position of any line on it is not 
greater than 1 millimetre. 
Argon resembles nitrogen in that it gives two distinct spectra according to the 
strength of the induction current employed. But while the two spectra of nitrogen 
are different in character, one showing fluted bands and the other sharp lines, both 
the argon spectra consist of sharp lines. It is, however, very difficult to get argon 
so free from nitrogen that it will not show the nitrogen flutings superposed on its own 
special system of lines. I have used argon prepared by Lord Rayueicu, Professor 
Ramsay, and myself, and, however free it was supposed to be from nitrogen, I 
could always at first detect the nitrogen bands in its spectrum. ‘These, however, 
disappear when the induction spark is passed through the tube for some time, 
varying from a few minutes to a few hours. The vacuum tubes best adapted for 
showing the spectra are of the ordinary Pliicker form having a capillary tube in the 
middle. For photographing the higher rays which are cut off by glass I have used a 
similar tube, “end on,” having a quartz window at one end. I have also used a 
Pliicker tube made entirely of quartz worked before the oxy-hydrogen blow-pipe. I 
have not yet succeeded in melting platinum or iridio-platinum wire terminals into 
the quartz, as they melt too easily, but a very good spectrum is obtained by coating 
the bulbs outside with tin foil, connected with the terminals of the induction coil. 
The pressure of argon giving the greatest luminosity and most brilliant spectrum 
is 3 millims. At this point the colour of the discharge is an orange-red, and 
the spectrum is rich in red rays, two being especially prominent at wave-lengths 696°56 
and 705°64. On passing the current the traces of nitrogen bands disappear, and 
the argon spectrum is seen in a state of purity. At this pressure the platinum from 
the poles spatters over the glass of the bulbs, owing to what I have called ‘“ electrical 
MDCCCXCV.—A, 2a 27.6,95, 
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