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.
I have now finished this imperfect account. It is but an apology for not
having brought the process itself before you. I have done the best I could
under the circumstances; and I know your kindness well, for if I were not
aware that I might trust to it, I would not appear here so often as I have
done. The gradual loss of memory and of my other faculties is making
itself painfully evident to me, and requires, every time I appear before
you, the continued remembrance of your kindness to enable me to get
through my task. If I should happen to go on too long, or should fail in
doing what you might desire, remember it is yourselves who are chargeable,
by wishing me to remain. I have desired to retire, as I think every man
ought to do before his faculties become impaired; but I must confess that
the affection I have for this place, and for those who frequent this
place, is such, that I hardly know when the proper time has arrived.
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