580 
MESSRS. W. SPOTT1SWOODE AND J. FLETCHER MOULTON 
reasoning would not apply without great modifications. It is true that there is a 
similar contrast between discharges which give strong shocks and those which do not 
do so, but that may arise from wholly different considerations, which will be noticed in 
due time (see Section XXVII.). It is partly on account of the great importance of 
distinguishing between this latter phenomenon, which is peculiar to high-vacuum 
tubes, and the phenomenon to which this section has chiefly been dedicated, which is 
common to all tubes, that has made us examine the latter. 
XA r II.— On the phenomenon of phosphorescence in vacuum tubes. 
Before entering into the investigation of vacuum discharges through tubes of high 
exhaust, it will be advisable to consider from all points of view a phenomenon which is 
a very marked accompaniment of discharges in high vacua, though, as we shall see, by 
no means confined to them. We allude to the well-known phosphorescence which 
appears on the inside of the tube, especially in the neighbourhood of the negative 
terminal. This phenomenon required only incidental notice in our former paper, not 
because it does not belong to the sensitive state (for we shall find that it is quite as 
marked a feature in the case of the intermittent as in that of the continuous discharge), 
but because it is of comparatively rare occurrence in connexion with the low exhausts 
to which our researches were there confined. But the remainder of the present paper 
will be chiefly devoted to the consideration of discharges in high vacua, and it is 
therefore necessary that we should start with a clear understanding of the nature and 
laws of this phenomenon. 
The immediate cause of phosphorescence in vacuum tubes is ascertained beyond 
controversy. Goldstein and Crookes have showm that it arises from streams of 
molecides or small particles of some kind which pour off from the negative terminal 
during the continuance of the discharge. These while moving at high velocities strike 
against the glass and by their impact impart sufficient energy to the glass to render it 
luminous, and also to raise its temperature very considerably. The peculiar colour of 
the light thus generated has been conclusively shown to depend solely on the com¬ 
position of the glass ; it is indifferent to the substance of which the terminal is 
composed; in fact, as will be seen later, the very glass itself may serve as such a 
terminal. Its configuration undoubtedly depends upon that of the terminal as well 
as upon that of the surface of the glass upon which it is actually formed; but as the 
phosphorescent light has no direct connexion with the luminosity of the gas itself, it 
gives us no direct information as to what is going on within the tube, save so far as it 
testifies to the existence of these streams of material particles coming from the 
negative terminal. 
The relation of this to other electrical phenomena seems not to Lave been clearly 
understood, and it has been supposed by some to indicate that the gas within the 
tube is in some special and peculiar state differing widely from the ordinary gaseous 
