1895.] Note on the Spectrum of Argon. 347 



natural to borrow the first letter of Lord Rayleigh's and Professor 

 Ramsay's names to give to the unknown lines, and in the measure- 

 ments of the photographs which showed the lines well, there appears 

 an "R" against seventeen lines out of sixty-one measured, the re- 

 maining lines being known to belong to Hg, H, N, and nitrocarbons. 

 It transpires now, as I learnt from reading the abstract of the paper 

 in which Lord Rayleigh and Professor Ramsay describe their con- 

 summate researches on argon, that the symbol " A " should have been 

 used instead of " R " to designate the lines on my photographs. For 

 the lines are Argon lines. 



The conditions under which the spectrum of argon has appeared 

 in my investigations are of interest at the present time, and I hope a 

 description of them may not be unacceptable. 



A glass bulb was sealed hermetically to a mercury pump of the 

 Hagen-Topler form, in which there was strong sulphuric acid floating 

 on the surface of the mercury. The bulb was exhausted as low as 

 possible and refilled with air. The pressure was reduced to about 

 180 Millionths of an atmosphere (= 0'14 mm.), at which pressure a 

 bright discharge could be passed through the residual gases by means 

 of Professor J. J. Thomson's method of surrounding the bulb by a 

 coil of wire, which carries a very rapidly alternating current produced 

 by the discharge of a condenser. 



The discharge was passed for thirty minutes, during which time a 

 photograph of the spectrum was taken. The pressure of the gas in 

 the bulb fell during the passage of the discharge from the value 

 174 M (= 0-13 mm.) to 112 M (= 0'085 mm.). The spectrum shows 

 the bands of nitrogen strong, also mercury lines and nitrocarbon 

 groups strong, hydrogen weak, no oxygen or argon. 



Again the discharge was passed for thirty minutes and a new pho- 

 tograph was taken. The pressure fell from 100 M (= 0'076mm.) to 

 20 M (= 0'015 mm.) ; the nitrogen spectrum had faded considerably, 

 and there had appeared a number of fine lines which I was unable, 

 in spite of careful efforts, to identify with the lines of any known 

 substances. 



The nature of my method of investigation of spectra is such that 

 it is not difficult to pick out of the numerous spectra which appear 

 superposed on the photographic plate, the lines which belong to any 

 one spectrum. [The photographic reproductions sent herewith show 

 clearly the ease with which this may be done.] 



The results of measurement made in the last few days of seventy- 

 two lines in my " low-pressure spectrum " are given below, and side 

 by side are given the measurements of the wave-lengths determined 

 by Mr. Crookes for the argon lines. 



The agreement of the measurements shows conclusively that we 

 have been measuring the same spectrum. Between H y and wave- 



YOL. LVII. 2 c 



