OX THE SENSITIVE STATE OF VACUUM DISCHARGES. 
601 
with a positive as well as with a negative air-spark. In the former case the discharge 
at the negative terminal is of the nature of a response, and is presumably of a much 
less sharp and impulsive type than it is when that terminal is the air-spark terminal. 
Yet so brief is the whole time occupied by a single discharge that even the time 
required for the less instantaneous response is not sufficient to give any perceptible 
broadening out of the bands in the revolving mirror. It may, however, be said with 
much justice that it is probable that it is only the first burst of the response that is 
intense enough to give phosphorescence, and that the absence of continuous phos¬ 
phorescence in this case is no proof that there is not a gentle continuous discharge 
from what we have previously called the connected terminal. But even if we allow 
for this it is clear that the main part of the discharge from the negative terminal in 
the case of a positive air-spark must take place in an extremely short space of time. 
Although phosphorescence is intermittent, like all the other luminous phenomena of 
the sensitive discharge, there are many reasons which make it desirable to treat it 
separately from the luminous discharge. It is not, like strife or the negative glow, a 
part of the phenomena of gaseous discharge properly so called, but it is the effect of a 
mechanical radiation which accompanies the discharge. It is a diagram, a projected 
image of that radiation upon the surface of the glass ; while itself, it is the quasi¬ 
accidental effect of the fact that glass is the most convenient substance for transparent 
tubes, and that glass is usually made of substances which will give such phosphor¬ 
escence when exposed to streams of rapidly moving particles such as those in question. 
Hence it is only a secondary phenomenon of gaseous discharge, and for clearness it 
will be well to treat it quite separately, and as in no way a part of the luminosity due 
to the discharge. But just as the relief and special effects in the luminous discharge 
are due to interference with the actual main discharge by means of artificially produced 
discharges of like period, so it is found that the phosphorescence which is due to the 
main discharge is capable of being affected in a similar way, though, of course, the 
results are wholly different to those special and relief-effects of which we treated in 
our former paper, and which relate to the luminous column itself, i.e., to phenomena 
existing in the gaseous medium through which the discharge passes. And as phos¬ 
phorescence is the most marked of all the phenomena that accompany the discharge in 
high-vacuum tubes, we shall, in studying the sensitive state in high vacua, and the 
nature and circumstances of discharges therein, pay special attention to the phenomena 
that depend on phosphorescence, and shall in the subsequent sections examine these 
first, and subsequently proceed to consider those phenomena which are the true 
analogues of the luminous appearances in tubes of lower exhaustion. 
