The Chemical History of a Candle by Michael Faraday (the red fox clan .txt) 📖

fumes that rise from it; and even if it was so far enclosed 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.

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.

NOTES.

[Footnote 1: Page 16. The Royal George sunk at Spithead on The 29th of August, 1782. Colonel Pasley commenced operations for the removal of the wreck by the explosion of gunpowder, in August, 1839. The candle which Professor Faraday exhibited must therefore have been exposed to the action of salt water for upwards of fifty-seven years.]

[Footnote 2: Page 17. The fat or tallow consists of a chemical combination of fatty acids with glycerine. The lime unites with the palmitic, oleic, and stearic acids, and separates the glycerine. After washing, the insoluble lime soap is decomposed with hot dilute sulphuric acid. The melted fatty acids thus rise as an oil to the surface, when they are decanted. They are again washed and cast into thin plates, which, when cold, are placed between layers of cocoa-nut matting, and submitted to intense hydraulic pressure. In this way the soft oleic acid is squeezed out, whilst the hard palmitic and stearic acids remain. These are further purified by pressure at a higher temperature, and washing in warm dilute sulphuric acid, when they are ready to be made into candles. These acids are harder and whiter than the fats from which they were obtained, whilst at the same time they are cleaner and more combustible.]

[Footnote 3: Page 19. A little borax or phosphorus salt is sometimes added, in order to make the ash fusible.]

[Footnote 4: Page 27. Capillary attraction or repulsion is the cause which determines the ascent or descent of a fluid in a capillary tube. If a piece of thermometer tubing, open at each end, be plunged into water, the latter will instantly rise in the tube considerably above its external level. If, on the other hand, the tube be plunged into mercury, a repulsion instead of attraction will be exhibited, and the level of the mercury will be lower in the tube than it is outside.]

[Footnote 5: Page 29. The late Duke of Sussex was, we believe, the first to shew that a prawn might be washed upon this principle. If the tail, after pulling off the fan part, be placed in a tumbler of water, and the head be allowed to hang over the outside, the water will be sucked up the tail by capillary attraction, and will continue to run out through the head until the water in the glass has sunk so low that the tail ceases to dip into it.]

[Footnote 6: Page 37. The alcohol had chloride of copper dissolved in it: this produces a beautiful green flame.]

[Footnote 7: Page 54. Lycopodium is a yellowish powder found in the fruit of the club moss (Lycopodium clavatum). It is used in fireworks.]

[Footnote 8: Page 58. Bunsen has calculated that the temperature of the oxyhydrogen blowpipe is 8061° Centigrade. Hydrogen burning in air has a temperature of 3259° C., and coal-gas in air, 2350° C.]

[Footnote 9: Page 60. The following is the action of the sulphuric acid in inflaming the mixture of sulphuret of antimony and chlorate of potassa. A portion of the latter is decomposed by the sulphuric acid into oxide of chlorine, bisulphate of potassa, and perchlorate of potassa. The oxide of chlorine inflames the sulphuret of antimony, which is a combustible body, and the whole mass instantly bursts into flame.]

[Footnote 10: Page 63. The "air-burner," which is of such value in the laboratory, owes its advantage to this principle. It consists of a cylindrical metal chimney, covered at the top with a piece of rather coarse iron-wire gauze. This is supported over an argand burner, in such a manner that the gas may mix in the chimney with an amount of air sufficient to burn the carbon and hydrogen simultaneously, so that there may be no separation of carbon in the flame with consequent deposition of soot. The flame, being unable to pass through the wire gauze, burns in a steady, nearly invisible manner above.]

[Footnote 11: Page 74. Water is in its densest state at a temperature of 39.1° Fahrenheit]

[Footnote 12: Page 74. A mixture of salt and pounded ice reduces the temperature from 32° F. to zero—the ice at the same time becoming fluid.]

[Footnote 13: Page 82. Potassium, the metallic basis of potash, was discovered by Sir Humphrey Davy in 1807, who succeeded in separating it from potash by means of a powerful voltaic battery. Its great affinity for oxygen causes it to decompose water with evolution of hydrogen, which takes fire with the heat produced.]

[Footnote 14: Page 98. Professor Faraday has calculated that there is as much electricity required to decompose one grain of water as there is in a very powerful flash of lightning.]

[Footnote 15: Page 101. A solution of acetate of lead submitted to the action of the voltaic current, yields lead at the negative pole, and brown peroxide of lead at the positive pole. A solution of nitrate of silver, under the same circumstances, yields silver at the negative pole, and peroxide of silver at the positive pole.]

[Footnote 16: Page 129. The gas which is thus employed as a test for the presence of oxygen, is the binoxide of nitrogen, or nitrous oxide. It is a colourless gas, which, when brought in contact with oxygen, unites with it, forming hyponitric acid, the red gas referred to.]

[Footnote 17: Page 152. Marble is a compound of carbonic acid and lime. The muriatic acid being the stronger of the two, takes the place of the carbonic acid, which escapes as a gas, the residue forming muriate of lime or chloride of calcium.]

[Footnote 18: Page 186. Lead pyrophorus is made by heating dry tartrate of lead in a glass tube (closed at one end, and drawn out to a fine point at the other) until no more vapours are evolved. The open end of the tube is then to be sealed before the blowpipe. When the tube is broken and the contents shaken out into the air, they burn with a red flash.]

[Footnote 19: Page 216. Water-gas is formed by passing vapour of water over red-hot charcoal or coke. It is a mixture of hydrogen and carbonic oxide; each of which is an inflammable gas.]

Poster's note: "combustion that makes!" was corrected from a misprint "combusion that makes!" in the original.

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