1895.] Mr. W. Crookes. On the Spectra of Argon. 287 



The molecular weight of argon is therefore 39 '8, and the same 

 number expresses its atomic weight, unless it be a mixture of two 

 elements, or of mono- and diatomic molecules of the same element. 

 The ratio of specific heats might support the last supposition ; but 

 the thermal behaviour of the gas lends no support to this view. 



II. " On the Spectra of Argon." By WILLIAM CROOKES, 

 F.R.8., &c. Received January 26, 1895. 



(Abstract.) 



Through the kindness of Lord Rayleigh 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. 



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, the argon spectra both 

 consist of sharp lines. It is, however, very difficult to get argon so free 

 from nitrogen that it will not at first show the nitrogen flutiiigs super- 

 posed on its own special system of lines. I have used argon prepared 

 by Lord Rayleigh, Professor Ramsay, and myself, and, however free 

 it was supposed to be from nitrogen, I could always 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. 



The pressure of argon giving the greatest luminosity and most 

 brilliant spectrum is 3 mm. 



If the pressure is further reduced, and a Leyden jar intercalated 

 in the circuit, the colour of the luminous discharge changes from red 

 to a rich steel blue, and the spectrum shows an almost entirely dif- 

 ferent set of lines. 



I have taken photographs of the two spectra of argon partly super- 

 posed. In this way their dissimilarity is readily seen.* In the 

 spectrum of the blue glow I have counted 119 lines, and in that of 

 the red glow 80 lines, making 199 in all. Of these 26 appear to be 

 common to both spectra. 



I have said that the residual nitrogen is removed by sparking the 



* Photographs of the different spectra of argon, and other gaseous spectra for 

 comparison, were projected on the screen. 



