MR. W. CROOKES ON THE SPECTRA OF ARGON. 245 
taminated by a trace of nitrogen bands. The next day the tube with platinum 
terminals was unchanged, but that having aluminium terminals showed the pure spec- 
trum of argon, the faint nitrogen bands having entirely disappeared during the night. 
After an hour's current and a few days’ rest the tube with platinum terminals likewise 
gave a pure argon spectrum. When a mixture of argon with a very little nitrogen is 
submitted to the induced current in a tube made of fused and blown quartz, with- 
out inside metallic terminals, the nitrogen bands do not disappear from the argon 
spectrum, but the spectra of argon and nitrogen continue to be seen simultaneously. 
A yacuum-tube was filled with argon and kept on the pump while observations 
were made on the spectrum of the gas as exhaustion proceeded. The large coil was 
used with a current of 8°84 amperes and 11 volts, no jar being interposed. At a 
pressure of 3 millims. the spectrum was that of the pure red glow. This persisted as 
the exhaustion rose, until, at a pressure of about half a millimetre, flashes of blue 
light made their appearance. At a quarter of a millimetre the colour of the ignited 
gas was pure blue, and the spectrum showed no trace of the red glow. 
A striking instance of a change of spectrum from nitrogen to argon was shown in 
a tube filled with argon kindly sent me by Lord RayteicH. It had been prepared 
from the atmosphere by sparking, and it was considered ‘to contain about 3 per cent. 
of nitrogen. This argon was passed into an exhausted tube and then rarefied to a 
pressure of 75 millims. and kept on the pump. At this pressure the nitrogen con- 
ducted all the induction current, the spectrum showing nothing but the nitrogen 
bands. The pump was slowly kept going and spectrum observations were con- 
tinuously taken. When the pressure fell to about 3 millims. a change came over 
the spectrum, the nitrogen bands disappeared, and the spectrum of argon took its 
place, the only contamination being a little aqueous vapour, due to my not having 
sufficiently dried the gas. I took photographs of the spectrum given by this tube in 
the two stages, one showing the pure nitrogen bands and the other the argon lines, 
each being compared with the spectrum of argon prepared by Professor Ramsay. 
Observations have shown that the spectra given by argon, obtained by the sparking 
method of Lord Ray.ercH and by the magnesium method of Professor Ramsay from 
the atmosphere, are identical. 
It was of interest to see how little argon could be detected in admixture with 
nitrogen by corabined pumping and passage of the current. Some argon prepared 
by myself,* having 60 to 70 per cent. of nitrogen with it, was put into a small tube 
furnished with large platinum terminals. Exhaustion was carried to 3 millims., and 
* When a current of 65 volts and 15 ampéres, alternating 130 times a second, is passed through the 
primary of my large coil, an arching flame, consisting of burning nitrogen, issues from each of the 
secondary poles, meeting in the middle. When once started, the poles can be drawn asunder till the 
fiame bridges across 212 millims. When the terminals are more than 46 millims. apart, the flame will 
not strike across. By enclosing this flame ir a reservoir over alkaline water and feeding it with air and 
oxygen I can burn up a litre of air an hour. — 
