XXI
329
PASSAGE OF CATHODE RAYS THROUGH METALS
penetrates through the gold. But as the exhaustion increases
there is less and less light inside the discharge tube, and the
rays which impinge upon the glass are more purely cathode
rays. The glass now begins to phosphoresce behind the layer
of gold leaf, and when the cathode rays have attained their
most powerful development, the gold leaf, when observed from
the back, simply looks like a faint veil upon the glass plate,
chiefly recognisable at its edges and by the slight wrinkles in
it. It can scarcely be said to throw a real shadow. On the
other hand the thin mica plate, which we have superposed on
the gold leaf, throws through this latter a marked black
shadow upon the glass.
Thus the cathode rays seem to pene-
trate with but little loss through the layer of gold. I have
tested other metals in the same way with the same result-
silver leaf, aluminium leaf, various kinds of impure silver and
gold leaf (alloys of tin, zinc, and copper), silver chemically
precipitated, and also layers of silver, platinum and copper
precipitated by the discharge in vacuo. These latter layers
were much thinner than the beaten metallic leaves. I have
not observed any characteristic differences between the various
metals. Commercial aluminium leaf seems to work best. It
is almost completely opaque to light but very transparent to
the cathode rays: it is easily handled, and is not attacked by
the cathode rays, whereas a layer of silver leaf, for example, is
soon corroded by them in a peculiar manner.
It might be urged, against the assumption that the
cathode rays in these experiments penetrate right through the
mass of the metal, that such thin metallic layers are full of
small holes, and that the cathode rays might well reach the
glass through these without going through the metal. It is
the behaviour of the beaten metallic leaves that is most sur-
prising, and one is bound to admit that these contain many
pores but the aggregate area of the holes scarcely amounts
to a few per cent of the area of the leaf, and is not sufficient
to account for the brilliant luminescence of the covered glass.
Furthermore, the covered part of the glass exhibits no lumin-
escence when it is viewed from the front, i.e. from the side on
which the cathode is. Hence the cathode rays must have
reached the glass by a way which the light excited by them
cannot retrace; so that they cannot have reached the glass