622 
MESSRS. W. SPOT TISTVO ODE AND J. FLETCHER MOULTON 
carriers of negative electricity, and the experiments which we have just described seem 
to show almost conclusively that such is not the case. 
The property which, as we have just seen, conductors within the tube possess of 
becoming negative terminals and thus giving out negative electricity on all sides, seem 
to account for a peculiarity in the attendant phosphorescence which merits remark. 
If the hand be passed along the under side of the tube at a little distance from it, the 
line of attendant phosphorescence will be seen sharply and clearly defined along the 
top of the tube until the hand comes to a place where a conducting object is lying in 
the tube. The line of phosphorescence will there he split in two and become irregular 
in outline, and will join again beyond the spot where the conductor lies, thus enclosing 
within it a space with no phosphorescence. This experiment is also of use as showing- 
strong grounds for holding that the streams which produce the attendant phos¬ 
phorescence come from the opposite side of the tube. 
It will now be seen how special is the importance of the evidence given us by these 
molecular streams. Without the definite evidence which they give of the existence of 
sources of negative discharge, it would have been left to speculation to determine the 
nature of the action in the tube which accompanies a discharge. And as we have 
seen that these molecular streams to which phosphorescence is due are not peculiar to 
tubes of high exhaust, but probably exist as an accompaniment of negative discharge 
in tubes of all degrees of vacuum, we see that we have here a fresh step in the 
analysis of the mode of propagation of vacuum discharges in general. 
XXIV .—On the special effect in tubes of high exhaustion with a positive air-spark. 
We have seen in our former paper that the characteristic peculiarity of the special 
effect in tubes of low vacua with a positive air-spark is that the luminosity is attracted 
instead of repelled. If a wire from the positive terminal be carried parallel to the 
tube, and at a little distance from it, a line of luminosity will appear on the side of the 
tube nearest to the wire throughout its whole length. 
If the same experiment be tried in high vacua precisely the same effect is produced. 
The thin pencil-like column of which we have spoken in the last section will be found 
pressed close to the side of the tube nearest to the wire. It follows all the movements 
of the wire, and takes a curved direction when the wire does so. This alone would 
furnish a strong presumption of the radical identity of the modes of discharge in low 
and high vacua, were any further proof needed. 
There is, however, an apparent peculiarity in the behaviour of this thin pencil-like 
column, wdien the special effect is produced, which must be mentioned. When the piece 
of metal attached to the wire which is in connexion with the positive terminal is brought 
close to the tube there is no longer an attraction of the positive luminosity, but a strong 
repulsion (Plate 29, fig. 29). There is nothing in this phenomenon to invalidate the 
above conclusions. An exactly analogous phenomenon is observed in the case of tubes of 
