An Essay on Combustion, With a View to a New Art of Dying and Painting

CHAPTER I.. Reduction of Metals

REDUCTION OF METALS

HYDROGEN GAS.

As Hydrogen, or the base of inflammable air, seems to act an important part in the following experiments, and is, according to some chymists, pure phlogiston itself; I have therefore assigned the first chapter in this essay to hydrogen gas, this being the most simple form, in which hydrogen has hitherto been obtained.

But I had not the means of procuring this gas free of carbone, and other impurities, that it dissolves and elevates during its formation; or of excluding atmospheric air, which circumstances, I am sensible, must, in some measure, influence the result; as it cannot be supposed, that these extraneous bodies would remain inactive during the process of reduction. However, as such exactness is not in the power of every experimenter, I was under a necessity of using hydrogen gas, with the disadvantages here mentioned.

Though most of the experiments in this essay were made on a variety of white and coloured silks, I shall relate only those made on the former, as it exhibits the changes produced more distinctly, and contains less foreign matter to modify the result of experiment; and, for brevity’s sake, I shall use the term silk, by which white silk is always designed.

EXP. 1. GOLD.

I poured a quantity of sulphuric ether on a solution of gold in nitro-muriatic acid, and, by this means, obtained a solution of gold in ether, which I separated from the acid.

In this ethereal solution of gold, I dipped a piece of silk, after it was taken out, and the ether evaporated, it was suspended in a tall glass cylinder, placed over a vessel, containing a mixture of diluted sulphuric acid, and iron filings.

The silk thus exposed to a rapid current of hydrogen gas acquired, after some time, a purple colour, and a large spangle of gold, of an irregular figure, appeared on the upper end of the silk, and looked very brilliant.

The silk was kept exposed to the gas about four months, and frequently observed, but no other remarkable change was perceived, except that the purple colour became more intense: the experiment was now discontinued; and on taking out the silk, and examining it in the light of the sun, I observed many particles of reduced gold; but they were very small, and by no means so brilliant, as that, which first appeared.

During the experiment, fresh quantities of materials for supplying hydrogen gas were occasionally added.

From the brilliant spangle of reduced gold observed in this experiment, I concluded, that there was a sufficient quantity of the metal in the fibres of the silk, could it be reduced.

This spot of reduced gold was very permanent, and adhered firmly to the silk.

EXP. 2. GOLD.

In order to determine, whether a solution of gold in ether, or one in water, were best adapted to the object of these experiments; I evaporated to dryness a solution of gold in nitro-muriatic acid, and dissolved the salt in distilled water: in this solution, I immersed a piece of silk, which, after it was dried in the air, was suspended in a glass cylinder, like the former piece, and exposed to the action of hydrogen gas about two months.

The silk after some time assumed a purple colour, and five or six specks of reduced gold, of the size of pin heads, and one much larger, were observed. Examining the silk in the sun-beams, I perceived the whole of it spangled with minute particles of reduced gold.

After many experiments with these two solutions of gold, I was led to conclude, that the solution in water answered best.

EXP. 3. SILVER.

Having dissolved some pure silver in diluted nitric acid, and evaporated some of the water by a gentle heat, I placed it in the dark to crystallize. The crystals were separated from the solution, and dissolved in distilled water; to one measure of this solution, which was saturated with the nitrate of silver, ten or twelve measures of distilled water were added.

In this diluted solution, a piece of silk was dipped; after it was taken out, it was dried at the fire: the silk dried in this manner, retained its white colour; whereas, were it dried gradually in the air; and especially, were the light considerable, its white colour would be changed to a reddish brown, more or less intense, according to the quantity of light present.

Some kinds of silk, on being immersed in a solution of nitrate of silver, have some of their threads immediately coloured brown: but I often got white silk, which would retain its pure white colour, if dried at the fire, or in the dark.

The piece of silk dried, as above, was suspended in the middle glass of Dr. Nooth’s machine; and into the lower glass were introduced a diluted solution of sulphuric acid, and some small iron nails.

The machine was now placed in a dark closet, to exclude the action of light; after some time, the silk, thus exposed to hydrogen gas, put on a light brown colour, which gradually became deeper, until it was changed to a disagreeable black, with some mixture of brown: then very small particles of reduced silver began to appear, which increased by degrees, in number, and brightness.

The experiment was continued four months; after which time, the silk was taken out of the machine: it had a gray metallic appearance, intermixed with a considerable tinge of brown.

This experiment was repeated on another piece of silk, every circumstance being the same, except that a quantity of water was put in the middle glass of the machine, over which the silk was suspended; the intention of interposing the water in this experiment was to purify the gas.

The appearances were much the same, as in the last experiment; except that the silk became black sooner; and that some spangles of the silver were larger, and better reduced.

The specks of reduced silver were permanent, and adhered firmly to the silk.

EXP. 4. LEAD.

In a solution of acetite of lead in distilled water I dipped a piece of silk, and dried it in the air; it was then suspended over a quantity of water in Dr. Nooth’s machine, and exposed to hydrogen gas about three months.

The appearances were much the same as in the last experiment, except that the silk was not so black: the particles of reduced lead resembled silver.

Similar experiments were made in this machine on bits of silk imbued with a solution of nitro-muriate of gold in water; but the appearances being much the same as those mentioned in the first and second experiments, it is unnecessary to repeat them here.

It is much easier to imagine, than express my anxiety, to discover the cause of the brilliant specks of reduced gold and silver, with which the pieces of silk in these experiments were sometimes spangled; concluding, that were the cause ascertained, it would be a considerable step towards accomplishing the object in view.

But many and diversified were the experiments I made before the circumstance, on which these spangles depended, was perceived.

At length, I found it depended on the presence of moisture; and that the spangles of reduced metal were owing to small drops of water carried up by the elastic fluid, and deposited on the silk.

The experiments, which led to this conclusion, shall be related in their proper places. I shall now proceed to recite the remainder of the experiments in this essay, conformable to the plan laid down in the preface, premising an occurrence observed in the course of them.

I sometimes found, that the production of hydrogen gas from a mixture of acid, water, and iron nails, would unexpectedly stop, which induced me to add more acid; this not succeeding, I added more water; but still no gas was formed: the proportions of acid and water were varied without effect. Thinking the mixture of acid and water unfit for the purpose, I poured it off, and repeatedly washed and agitated the nails with more water, which was also poured off; but happening inadvertently to add some of the mixture of acid and water, which I had before poured off as unfit for use, I was surprised to find the gas produced with great rapidity.

Reflecting on the cause of this odd circumstance, I concluded that it depended on a crust of superoxygenated iron, formed on the surface of the nails, which defended them from the action of the water and acid, and that this crust was removed by the agitation and washing, which enabled the diluted acid to act again on the iron.

EXP. 5. GOLD.

I immersed a bit of silk in a solution of nitro-muriate of gold in distilled water, and dried it in the air; it was then placed over a cylindrical glass vessel, containing a mixture of diluted sulphuric acid and iron nails, for about half an hour; but no reduction of the metal could be observed.

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