ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
203 
towards the orifice of the tube (Plate 18, fig. 17).* And this phenomenon is not due 
to the shape of the sides of the glass; for it may be produced in a tube of uniform 
section. If we join to earth a ring of tinfoil placed round a large tube in which striae 
are formed by the use of a coil working with a high-speed break, and thus constrict 
the striae in the immediate neighbourhood of the ring, it will be found that those on 
the positive side of the ring have the same peculiarity as we have above noticed. 
Their bright surfaces are concave, but as you pass from the constricted portion each is 
of less curvature than the one before it. 
We see, then, that we can greatly modify the bright outline of a stria by modifying 
the size and shape of the next stria, i.e., of the negative terminal of this unit. If 
we make this small, then the bright surface of the next stria will curve round it and 
be concave; if, on the other hand, it is large, and still more if it be hollow and 
partially envelop the next stria, the bright surface of the latter will be very convex. 
Now if we replace this negative terminal (whose hazy, gaseous structure is readily 
capable of being moulded by and probably owes its form and structure to the discharge 
itself) by a fixed metallic body, we must expect great modifications in the appearance 
of the other members of the unit. It is not to be wondered at that the dark space, 
and the bright surface which forms the other boundary of this dark space, should 
follow the outlines of the new negative terminal, and that if it consist of a small 
metallic body projecting into the tube, the general appearance should be as if the 
bright surface enveloped it, remaining always separated from it by a dark space. This 
is exactly the appearance of the negative glow, and the above is, we believe, a com¬ 
plete explanation of its formation and function. This negative glow and the haze 
behind it, which terminates in what is known as the negative dark space, is, according 
to this theory, a stria turned inside out by the influence of the shape and character of 
the negative terminal (Plate 18, fig. 18).t 
We have only to reflect on the very remarkable way in which the negative glow 
shows the shape of the negative terminal, to see that there is nothing improbable in 
assuming that such a deformation has taken place. If the negative terminal be 
spherical, the negative glow forms a spherical envelope round it; if it be merely a 
* An exactly similar phenomenon is shown in the familiar experiment of the electric egg, where the 
striae which surround the negative terminal are concave on their bright surfaces. 
t It will doubtless occur to those familiar with striated discharges, that there are considerable differences 
in the behaviour of striae and the negative glow when subjected to the action of a magnet. It must, however, 
be remembered that the positions of the striae are dependent on one another, since each receives the dis¬ 
charge from the one behind it. But in the case of the negative glow the lines of electric discharge whose 
termination it marks are fixed to the negative terminal at one end, and thus the glow can only be displaced 
so far as the deformation of these lines will permit. In fact, the negative glow is like a stria at anchor. 
There is just the same difference between the behaviour of the two as between the effect of waves on float¬ 
ing seaweed and seaweed that is growing on rocks. In this respect the artificial striae of which we have 
been speaking resemble the negative glow, with the exception that they have a fixed positive terminal 
instead of a fixed negative terminal. 
2 D 2 
