from the air that you could not form any litharge, you would still have
those abundant fumes flying off. I may also take a piece of gold, and shew
you the same thing. I have here a piece of gold which I put upon a clean
surface of Paris limestone. Applying the heat of the blowpipe to it, you
see how the heat drives off the vapour; and if you notice at the end of
the Lecture, you will observe on the stone a purple patch of condensed
gold. Thus you see a proof of the volatilisation of gold. It is the same
with silver. You will not be startled if I sometimes use one agent and
sometimes another to illustrate a particular point. The volatility of gold
and silver is the same thing, whether it be effected by the voltaic
battery or by the blowpipe. [A lump of silver was placed in a charcoal
crucible between the poles of a voltaic battery.] Now, look at the fumes
of silver, and observe the peculiar and beautiful green colour which they
produce. We shall now shew you this same process of boiling the silver,
cast on a screen from the electric lamp which you have before you; and
while Dr. Tyndall is kindly getting the lamp ready for this purpose, let
me tell you that Deville proposes to throw out in this way all these
extraneous things that I have spoken of, except two--namely, iridium and
rhodium. It so happens, as he says, that iridium and rhodium do make the
metal more capable of resisting the attacks of acids than platinum itself.
Alloys are compounded up to 25 per cent. of rhodium and iridium, by which
the chemical inaction of the platinum is increased, and also its
malleability and other physical properties. [The image of the voltaic
discharge through vapour of silver was now thrown upon the screen.] What
you have now on the screen is an inverted image of what you saw when we
heated the silver before. The fine stream that you see around the silver
is the discharge of the electric force that takes place, giving you that
glorious green light which you see in the ray; and if Dr. Tyndall will
open the top of the lamp, you will see the quantity of fumes that will
come out of the aperture, shewing you at once the volatility of silver.