OF GASES AT HIGH EXHAUSTIONS. 
407 
just mentioned—have lost much of their interest. I will therefore only briefly mention 
some of the results at a few pressures. 
The poles, formed of aluminium, are 4 millims. apart at their ends. The coil when 
in full work gives a 6-inch spark, but a small battery was used in order to reduce the 
full striking distance in air to 85 millims. 
664. At an exhaustion of 295 M the resistance of 4 millims. of rarefied air in the 
tube between the poles is equal to 2 millims. outside ; that is to say, when the outer 
terminals of the coil are 2 millims. apart it is a matter of indifference whether the 
spark passes between them or through the vacuum tube. There is much reddish- 
violet light round the poles, the appearance being that of ordinary vacuum poles. 
Traces of green phosphorescence flash out occasionally. 
At an exhaustion of 82 M the violet light round the poles has disappeared. The 
+ end of the tube is filled with hazy violet light, and there is a small spark and a 
brush at the tip of the — pole. Faint green phosjfliorescence is seen on the glass 
round the — pole. The resistance of the vacuum is now equal to 24 millims. air 
space. 
Exhaustion 27 M. The violet diffused light is much fainter, and the green 
phosphorescence is getting stronger. Resistance of the vacuum = 24 millims. of air. 
Exhaustion 4 M. No violet light is to be seen. The green light is very strong, 
and a diffused greenish-yellow 7 light fills the + end of the tube. Resistance of the 
vacuum = 53 millims. of air. 
Exhaustion 1 M. The green phosphorescence is like the last, but somewhat 
stronger. The spark and brush at the tip of the — pole, first noticed at 82 M, 
are still there. Resistance of the vacuum =85 millims. of air space. 
Exhaustion 0'5 M. The appearance is similar to the last, but the spark and brush 
have gone from the — pole. The wires connecting the coil with the tube were now 
carefully insulated. When the poles are kept apart the striking distance in air (85 
millims.), the spark often strikes across rather than pass through the tube, but on 
separating the poles beyond this distance the discharge flashes intermittently through 
the tube, sometimes keeping up for one or two seconds. 
When thus passing through the tube the flashes of green phosphorescence are very 
brilliant, particularly at the + end. During the intervals between the flashes a faint 
band of green light is seen round the inner surface of the tube at the tip of the — 
pole. 
Exhaustion 0‘02 M. With the battery and coil as hitherto no discharge whatever 
passes. On increasing the battery power till the striking distance in air was 100 
millims. the spark occasionally passed through as an intermittent flash, bringing out 
faint green phosphorescence on the glass round the end of the — pole. 
665. The terminals of the tube and wires leading to them must be well insulated, 
and kept away from any part of the apparatus. If this precaution is neglected a 
spark breaks through from one of the wires to some part of pump or connecting tubes, 
