944 MR, W. CROOKES ON THE SPECTRA OF ARGON. 
evaporation,’* and I think the residual nitrogen is occluded by the finely-divided 
metal. Similar occlusions are frequently noticed by those who work much with 
vacuum-tubes. - 
If the pressure is further teal and 4 teen jax 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 different set of lines. The two spectra, called for 
brevity red and blue, are shown on the large map, the upper spectrum being that of 
“blue” argon, and the lower one that of “red” argon. It is not easy to obtain the 
blue colour and spectrum entirely free from the red. The red is easily got by using 
a large coilt actuated with a current of 3 ampéres and 6 volts. There is then no 
tendency for it to turn blue. The blue colour may be obtained with the same coil by 
actuating it with a current of 3°84 ampéres and 11 volts, intercalating a jar of 
50 square inches surface ; the make-and-break must be screwed up so as to vibrate 
as rapidly as possible. With the small coil a very good blue colour can be obtained by 
using three Grove'’s cells and a Leyden jar of 120 square inches surface, and a very 
rapid make-and-break. It appears that an electromotive force of 27,600 volts is 
required to bring out the red, and a higher E.M.F. and a very hot spark for the blue. 
It is possible so to adjust the pressure of gas in the tube that a very slight alteration 
of the strength of the current will cause the colour to change from red to blue, and 
vice versd. I have occasionally had an argon tube in so sensitive a state, that 
with the commutator turned one way the colour was red, and the other way 
blue. Induction coils actuated by a continuous current are never symmetrical as 
regards the polarity of the induced current, and any little irregularity in the metallic 
terminals of the vacuum-tube also acts as a valve. The red glow is produced by the 
positive spark and the blue by the negative spark. 
I have taken photographs of the two spectra of argon partly superposed. 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 tube for some 
time when platinum terminals are sealed in. This is not the ouly way of purifying 
the argon. By the kindness of Professor Ramsay, I was allowed to take some vacuum 
tubes to his laboratory and there exhaust and fill them with some of his purest argon. 
On this occasion I simultaneously filled, exhausted, and sealed off two Pliicker tubes, 
one having platinum and the other aluminium terminals. On testing the gas 
immediately after they were sealed off, each tube showed the argon spectrum, con- 
* “Roy. Soc. Proc.,’ vol. 50, p. 88, June, 1891. 
+ The coil used has about 60 miles of secondary wire, and when fully charged gives a torrent of sparks 
24 HGS: long. The smaller coil gives six-inch sparks when worked with six half-pint Grove’s cells. 
¢ Photographs of the different spectra of argon, and other gaseous spectra for Conese were 
nboiseiod on the screen. 
