ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
585 
pressure, and giving an amorphous positive column, the exhaust not being sufficiently 
high to give stratification. Every one of these readily gave phosphorescence under 
any one of the foregoing methods. As it was of course unnecessary to examine tubes 
of higher exhaust, we then set to work to examine tubes of lower exhaust. Of these 
we had but few instances; for if the exhaust be much less than that described above, 
the special phenomena of vacuum discharges are very imperfectly manifested, and 
hence such tubes would be useless as vacuum tubes. One, however, was found about 
an inch and three-quarters in diameter, containing vapour of bromine, in which the 
exhaust was so moderate that the luminous discharge consisted of a thin red line 
extending from the positive terminal up to within a very short distance of the 
negative. This, on being subjected to the last of the tests above described, gave 
splendid phosphorescence. 
By way of a crucial experiment a tube of about L inch external diameter was taken 
which had a cavity at one end filled with potash. In its ordinary state this tube is one 
of very great resistance, and displays splendid phosphorescence throughout almost its 
whole length (Plate 26, fig. 9). By heating the potash with a spirit lamp gas is driven 
out from it into the tube, and thus the degree of the exhaust can be lowered to any 
desired extent. This was subjected to the method last described, and the potash was 
heated until it melted, when we were compelled to desist from fears for the safety of 
the tube. Very bright green phosphorescence was manifested throughout the whole of 
the time, and it was clear that we had not yet reached the limits of pressure at which 
it could have been obtained. It was of course difficult to estimate exactly the pressure 
of the gas in the tube at the termination of the experiment, but from the resistance of 
the tube, and the appearance it gave when a current was sent through it, we judged 
it to be equal to at least lialf-an-inch of mercury. 
Feeling the importance of demonstrating conclusively that these molecular streams 
are not dependent on the existence of any specially high state of exhaust, we next 
took a tube of about 2 inches diameter which contained nitrogen at a pressure of 
about 2 millims., and permitted air to enter slowly through the stopcock which closed 
it. The arrangement for producing the phosphorescence was that last described. We 
found that it gave marked phosphorescence until air had been entering for a consider¬ 
able time. When at length the pressure became so great that no phosphorescence 
appeared, we examined the tube by passing a current through it, and found that it 
gave no luminous phenomena save in the immediate neighbourhood of the two ter¬ 
minals : an appearance which is well known to signify a very moderate exhaust. The 
phosphorescence faded gradually as the air entered, and when at last we decided that 
the phosphorescence had disappeared, it was merely because it had faded to such an 
extent that we could no longer certainly recognise its presence by the eye. But there 
was no sudden or discontinuous change marking the exact epoch of its disappearance, 
nor was there anything to lead us to believe that there had been any sudden cessation 
of its existence at the moment when it ceased to be visible. 
