246 MR. W. CROOKES ON THE SPECTRA OF ARGON. 
the tube was then sealed off. The spark from the large coil, actuated with a current 
of 3°84 amperes and 11 volts, was then put on, and the spectrum examined con- 
tinuously. At first it showed only the nitrogen bands; in about half an hour the 
nitrogen began to fade and the argon lines appeared, and in a few minutes later the 
tube was just short of non-conducting. The colour of the gas was rich steel blue, 
and the spectrum was that of the blue argon glow. Here the small diameter of 
the bulbs of the tube and the large platinum wires facilitated much spattering 
” of the platinum; the pressure also was the one most 
suitable for that phenomenon. To this I attribute the rapid occlusion of the 
residual nitrogen. 
An experiment was now made to see if the small quantity of argon normally present 
in the atmosphere could be detected without previous concentration. Nitrogen was 
prepared from the atmosphere by burning phosphorus, and was purified in the usual 
manner. This gas, well dried over phosphoric anhydride, was passed into a vacuum 
tube, the air washed out by two fillings and exhaustions, and the tube was finally 
sealed off at a pressure of 52 millims. It was used for photographing the band 
spectrum of nitrogen on several occasions, and altogether it was exposed to the 
induction current from the large coil for eight hours before any change was noticed. 
The last time I used it for photographing the nitrogen spectrum difficulty was 
experienced in getting the spark to pass, so I increased the current and intercalated a 
small jar. The colour immediately changed from the reddish-yellow of nitrogen to the 
blue of argon, and on applying the spectroscope the lines of argon shone out with 
scarcely any admixture of nitrogen bands.. With great difficulty, and by employing 
a very small jar, I was able to take one photograph of this changed spectrum and 
compare it with the spectrum of argon from Professor Ramsay, both being taken on 
the same plate, but the tube soon became non-conducting, and I could not then force 
a spark through except by employing a dangerously large current. Whenever a 
flash passed it was of a deep blue colour. Assuming that the atmosphere contains 
1 per cent. of argon, the 3 millims. of nitrogen originally in the tube would contain 
0°03 millim. of argon. After the nitrogen had been occluded by the spattered 
platinum this pressure of argon would be near the point of non-conduction, 
or “electrical evaporation 
In all cases, when argon has been obtained in this manner, the spectrum has Bean 
that of the blue-glowing gas. Very little of the red rays can be seen. ‘The change 
from red to blue is chiefly dependent on the strength and heat of the spark ; partly 
also on the degree of exhaustion. Nitrogen, when present, conducts the current 
easiest. As the exhaustion increases and the conductivity of the nitrogen diminishes, 
that of the red-glowing argon rises, until, at a pressure of about 3 millims., its 
conductivity is at the greatest, and the luminosity is best. Beyond that point the 
conductivity of the red form seems to get less, and that of the blue form to increase, 
till the vacuum approaches a fraction of a millimetre, when further pumping soon 
renders it non-conducting. It is not improbable, and I understand that independent 
