silver in its place.
The are of silver is not to be distinguished from that of thallium; it
is not only green, but the same shade of green. Are they then alike?
Prismatic analysis enables us to answer the question. However
impossible it is to distinguish the one _colour_ from the other, it is
equally impossible to confound the _spectrum_ of incandescent
silver-vapour with that of thallium. In the case of silver, we have
two green bands instead of one.

If we add to the silver in our camera a bit of thallium, we shall
obtain the light of both metals. After waiting a little, we see that
the green of the thallium lies midway between the two greens of the
silver. Hence this similarity of colour.

But why have we to 'wait a little' before we see this effect? The
thallium band at first almost masks the silver bands by its superior
brightness. Indeed, the silver bands have wonderfully degenerated
since the bit of thallium was put in, and for a reason worth knowing.
It is the _resistance_ offered to the passage of the electric current
from carbon to carbon, that calls forth the power of the current to
produce heat. If the resistance were materially lessened, the heat
would be materially lessened; and if all resistance were abolished,
there would be no heat at all. Now, thallium is a much more fusible
and vaporizable metal than silver; and its vapour facilitates the
passage of the electricity to such a degree, as to render the current
almost incompetent to vaporize the more refractory silver. But the
thallium is gradually consumed; its vapour diminishes, the resistance
rises, until finally you see the two silver bands as brilliant as they
were at first.[24]

We have in these bands a perfectly unalterable characteristic of the
two metals. You never get other bands than these two green ones from
the silver, never other than the single green band from the thallium,
never other than the three green bands from the mixture of both
metals. Every known metal has its own particular bands, and in no
known case are the bands of two different metals alike in
refrangibility. It follows, therefore, that these spectra may be made
a sure test for the presence or absence of any particular metal. If we
pass from the metals to their alloys, we find no confusion. Copper
gives green bands; zinc gives blue and red bands; brass--an alloy of
copper and zinc--gives the bands of both metals, perfectly unaltered
in position or character.

But we ar