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 are not confined to the metals themselves; the salts of these metals yield the bands of the metals.  Chemical union is ruptured by a sufficiently high heat; the vapour of the metal is set free, and it yields its characteristic bands.  The chlorides of the metals are particularly suitable for experiments of this character.  Common salt, for example, is a compound of chlorine and sodium; in the electric lamp it yields the spectrum of the metal sodium.  The chlorides of copper, lithium, and strontium yield, in like manner, the bands of these metals.

When, therefore, Bunsen and Kirchhoff, the illustrious founders of spectrum analysis, after having established by an exhaustive examination the spectra of all known substances, discovered a spectrum containing bands different from any known bands, they immediately inferred the existence of a new metal.  They were operating at the time upon a residue, obtained by evaporating one of the mineral waters of Germany.