542 
NATURE 
not the current moving freely in the gaseous space, upon which 
the magnet acts. 
This explanation of the magnetic displacement of a discharge 
receives strong support from the phenomena represented in Figs. 
5,6, ard 7. The successive bright lines there shown must be 
due to successive falls and revivals of tension within a single 
coil discharge. The existence of such alternations in coil dis- 
charges of large quantity is ctherwise known, When the fall 
in temperature is such that the conductivity of the gas is insvffi- 
cient to maintain the arc, the discharge can make its way through 
the air only by a fresh rent of the same kind as the first fracture. 
But how can this be reconciled with the fact that the tension can 
never reach its original degree, and must, on the whole, be 
gradually falling, and that, in addition, the paths represented 
by these various sparks are successively longer and longer? The 
answer to this question is to be found plainly written in the phe- 
nomenathemselyes. Any irregularity in one of these bright lines 
is always to be found accurately repeated in all of the same series. 
Now, it is scarcely to be conceived that, at successive instants of 
time and in different portions of space, irregularities in the dis- 
charge itself, and in the distribution of the gas, so precisely the 
same, would constantly and for certain recur; and we are there- 
fore driven to the conclusion, that it is the same portion of gas 
which at first cccupied the centre of the field, with its same, yet 
unhealed rent, which is moved outward under the action of the 
magnet. If this be so, we bave in this refetition of minute 
details, nothing more than what would necessarily follow from 
successive reopenings of the weak parts of the gas, which would 
be surely found out by the electricity in its struggle to pass. 
The view here taken of the material character of the luminous 
discharge is further borne out by the fact that the spindle of 
light is capable of being diverted by a blast of air. When the 
blast is gentle, the discharge becomes curvilinear, approximately 
semicircular, and the yellow flame may be seen playing abcut 
the outer edge in the same way as in a weak magnetic field. 
When the blast is stronger, the sheet of light becomes irregular 
in form, and it is traversed by a series of bright lines, all of 
which follow, even in their minute details, the configuration of 
Fic. 7. 
the sheet. The analcgy between this and the phencmena pro- 
duced in a strong magnetic field needs no further remark. If 
the strength of the blast be still further increased, the flame and 
the sheet of light both disappear, and nothing remains but 
bright sparks passing directly, and undisturbed, between the 
terminals. In this case the air is both displaced and cooled so 
rapidly by the blast, that it no longer offers a practicable con- 
ductive path for the remainder cf the electricity, coming from 
the coil, to follow. Of this a succesion of disruptive sparks is a 
necessary consequence. 
The effect thus produced by a very strong blast is in fact 
similar to that observed when a jar is used as a secondary con- 
denser. In this case the electricity, instead of flowing gradually 
from the coil, passes in one or more instantaneous discharges 
with finite intervals of time between them. Each of these has 
to break its way through the air; and that done, it ceases. 
Hence, neither a magnet, nor a blast of air will have any effect 
in diverting such a discharge. 
As a last stage of the phenomena, it may be mentioned that, 
if the interval between the terminals be near the limit of striking 
distance, either a blast of air, or the setting up of a magnetic 
field, will alike extinguish the discharge. 
Our experiments have been thus far carried on in air at atmo- 
spheric pressure ; | ut there is nothing in this pressure which is 
essential to them or to the conclusions to which we have been 
led. We may therefore repeat them in air, or any other gaseous; 
medium, at any pressure we please. This consideration leads 
us into the region (so fertile in an experimental point of view) 
of discharges in vacuum tubes. 
Commencing with a tube of mederate diameter and of very 
slight exhaustion, we can at once recognise our former pheno- 
mena slightly changed, Proceeding to another tube, of larger 
diameter and of mederate exhaustion, ard placing it axially or 
equatorially in a magnetic field, we see not only that the dis 
charge (or rather the conductor carrying it) is displaced, but also 
that the displaced part is spread out into a sheet or ribbon, 
showing that the discharge is affected gradually, exactly in the 
same way as was found in the open air. 
When the exhaustion is carried further, the } henomena become 
rather more complicated. At an early stage there is a distinct 
separation between the “‘ negative glow” and the rest of the 
luminous column ; and at a more advanced stage the column 
itself is broken into separate luminosities or stria. When this 
is the case, it is usually said that the negative glow follows the 
lines of magnetic force, while the luminous column distributes 
itself according to Amrére’s law. 
It will, however, be found that when completely analysed the 
action of the magnet upcn the stria, taken individually, is the 
same as that upon the negative glow, due allowance being made 
for the differences in local circumstances subsisting between the 
one and the other. We eave elsewhere shown that the negative 
glow is in reality as truly a stria as any other individual member 
of the luminous column; but with this difference, that it is 
anchored to, and dependent for its form on, a rigid metallic 
terminal, whereas each of the others is dependent on the variatle- 
form and position of the stria immediately next in order, reckon- 
ing from the negative end of the tube. The action of a magnet 
in throwing the negative glow into a sheet of light, which is the 
locus of the lines of force passing thrcugh the terminal, and 
which consequently varies with the position of the tube in the 
field, is a phenomenon so well known that we need repeat only 
a single experiment by way of reminder. 
Although it is not altogether so easy to show that the other 
striz are directly affected by a magnetic field in the same way as 
is the anchored stria, we may still satisfy ourselves that it is the 
fact, from the consideration that when the strize are well developed 
and the magnetic field is strong, it is quite possible to form a 
magnetic arch at any part of the column. In this experiment it 
will be noticed that for the formation of the arch in mid-column 
it is necessary that both poles of the magnet should act upon 
one and the same stria. This, in fact, means that the pole 
nearest the negative end anchors the stria, and thereby brings it 
into conditicns similar to those of the negative glow. When 
this is effected the two exhibit similar modifications in the 
magnetic field, 
In support of this view, we may adduce another and quite 
independent method of anchoring a stria, and of thereby pro- 
ducing a magretic arch elsewhere than at the negative terminal. — 
It was noticed by Goldstein and others that if the negative 
terminal of a tute be enveloped by an insulaing surface of any — 
form pierced with a number of holes or if a diaphragm simi- 
larly pierced be placed anywhere in the tube, that the pierced 
surface will act as a negative terminal. He also found that the 
finer and closer the holes, the more complete the resemblance to 
the action of a negative terminal, But even when the substance 
is metallic, and when the holes are neither very small nor very 
numerous, a perforated diaphragm will so far act like a negative 
terminal as to serve as a point of departure of a stria. There is, 
however, this difference, that the blank space immediately ad- 
joining the diaphragm, as it is usually called, is not generally so 
Jarge as that at the true terminal; and the strize thus artificially 
formed always lie close up to the holes. The diaphragm, in 
fact, anchors the stria, end renders it susceptible of the same 
magnetic effect as was shown in the cases studied before. 
‘The action of a diaphragm in a magnetic field gives rise to 
many other interesting and remarkable results; some of which 
would further illustrate the views now submitted for your con- 
sideration. But these must be reserved for another occasion. 
In the foregoing experiments, and in the remarks which have 
accompanied them, I have endeavoured to illu-trate, by reference 
to gaseous media, the principle enunciated at the outset, that in 
the displacement of the discharge in a magnetic field, the subject 
of the magnetic action is the material substance or medium 
which conveys the discharge. I have shown also that, even 
when the discharge takes place in media so attenuated as to 
produce the phenomena of stria, the same principle applies not 
only to the discharge as a whole, but also to each component 
stria or unit ; and, lastly, that the apparent diversity of effect cn 
the various striz is due to local circumstances, and not to any 
fundamental difference between the ‘‘ negative glow” and the 
members of the ‘‘ positive column.” : 
Seeing now that the magnetic displacement of the luminous 
discharge means displacement of the matter in a luminous con- 
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