ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
599 
a portion of the evidence in favour of the proposition of which we are treating. 
There are phenomena of the relief and special effects in tubes of high exhaust, which 
when we come to consider them will afford, if possible, still more conclusive evidence of 
the truth of the hypothesis. But it is for many reasons desirable that we should 
not anticipate the examination of the relief and special phenomena of high exhaust 
tubes, because we propose to deal with them separately as they merit a much more 
minute examination than we could conveniently give them in this section. Moreover, 
in our opinion, the tests and other experimental proofs that we have already given, 
and the complete absence of any phenomena which would lead us to believe in any 
breach of continuity in passing from the vacua used in our previous experiments to 
these high vacua, sufficiently establish the proposition that the discharge is in general 
carried along the tube by electricity of the same sign as that of the air-spark. 
XX.— Phosphorescence exists in sensitive as well as in continuous discharges; and when 
occurring in sensitive discharges it is intermittent in a like manner with the other 
luminous effects. 
If a tube, which is exhausted to a degree sufficient to produce phosphorescence 
when a continuous discharge is made to pass through it, be placed in circuit with the 
terminals of a Holtz machine (Plate 26, fig. 8), and an air-spark be introduced into 
the circuit, the only effect that is produced upon the phosphorescence is that it grows 
brighter, and is sometimes slightly altered in its position and distribution. Sub¬ 
stantially, the phenomenon remains the same as before the introduction of the air- 
spark, thus showing that the intermittence of the discharge does not prevent the 
negative pole from sending off the streams of molecules to which this phosphorescence 
is due, but, on the contrary, favours its so doing. This is in accordance with what 
has already been stated in Section XVII. 
To the eye the phosphorescence of the intermittent or sensitive discharge is just as 
continuous as that of the continuous discharge. But this is easily proved to be a 
mere optical effect, and that the phosphorescence is, in fact, like the discharge itself, 
intermittent. To demonstrate this, it is only necessary to take a revolving mirror 
and examine the tube through a narrow slit in the ordinary way. Whether there be 
a positive or a negative air-spark it will be found that the green luminosity is inter¬ 
mittent. If the air-spark be large a very small velocity of rotation will show the green 
lines which are the images of the slit clearly separated by dark bands ; and with an 
increased velocity of rotation the intermittence can clearly be shown even when the 
air-spark is very small. 
It might be thought that since phosphorescence is supposed to last for a short time 
after the excitement which causes it has ceased, the green light would be continuous 
even though its cause were intermittent. And there are traces of this when the very 
bright portions of the glass are examined with the revolving mirror, when the air- 
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