HKB
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. 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;