An essay on combustion, with a view to a new art of dying and painting

The next substance, I shall treat of, is Light, which seems well adapted to illustrate the theory of metallic reduction; as in experiments made with this substance, water can be more effectually excluded, than in trials with any other reducer of the metals.

Before any decisive experiments could be made on light, or indeed any other reducer of the metals, it is obvious that we should first ascertain the effects of water itself in the ordinary temperature of the atmosphere: to determine this point, the two following experiments were made.

Exp. 1. Gold.

A piece of silk was immersed in an aqueous solution of nitro-muriate of gold, and suspended in a phial over water: the phial was corked, covered with black silk, and placed in a dark closet, to prevent the action of light. The experiment was continued from the 20th of July to the 20th of October, during which time the silk was frequently observed, and found to continue moist: but no alteration could be perceived, except that the yellow colour, which the solution of gold gave the silk, was changed to a light brown.

The silk was now taken out of the phial, and a solution of phosphorus in ether applied to it: shortly after, a white metallic film appeared: the silk was then kept wet with water, and in some time, reduced gold of its proper colour appeared.

Exp. 2. Silver.

A bit of silk, which was dipped in an aqueous solution of nitrate of silver, was suspended in a phial over water, and placed in a dark closet, from the 20th of July to the 20th of October, and examined from time to time; but no alteration could be observed; its white colour continued pure, and unchanged.

The silk was now taken out of the phial, and the silver was immediately reduced by means of a solution of phosphorus in ether.

Hence it appears, that water alone has not the power of reducing metals in the ordinary temperature of the atmosphere.

Exp. 3. Gold.

On the 24th of July a piece of silk was immersed in a solution of nitro-muriate of gold in water, and dried by a gentle heat; it was then suspended in a window, exposed to the sunbeams, as much as possible: no change was perceived on it, till the 26th, when the margin of the silk began to assume a purple tinge, which increased gradually, and on the 29th exhibited a few obscure specks of reduced gold on the side of the silk opposed to the light.

The purple tinge continued to increase, and the yellow, which the solution of gold gave the silk, decreased gradually, till on the 27th of August the colour was a mixture of purple, and light brown.

The reduction of the gold seemed to keep pace with these changes of colour, and was very evident on some parts of the silk even in the gray light; but much more so in sunshine: these changes were observed till the 20th of October; the experiment was then discontinued; and scarce any reduced gold could be seen on the side of the silk not opposed to the light.

That this slight reduction of the gold, and changes of colour in the silk, depended on water absorbed from the air of the chambre, will appear from the following experiments.

Exp. 4. Gold.

A bit of silk, which was dipped in the solution of gold employed in the preceding experiment, was dried, and suspended in a crystal phial over dry carbonate of potash: the phial was then corked, and further secured from external humidity by covering the cork with wax: it was now placed in a dark closet 24 hours, that the carbonate of potash might have sufficient time to attract all the moisture, it could, from the air of the phial, before exposing it to the action of light.

The phial was then placed in a window, exposed, as much as possible, to the solar light, from the 24th of July to the 20th of October, and carefully observed: the same side of the silk was always opposed to the light, and had the yellow tinge, which the solution of gold gave, altered to a brownish yellow: but the other side of the silk suffered no visible change whatever; nor could any purple tint, or the smallest vestige of reduced gold be perceived.

Another bit of silk, which was immersed in the same solution of gold, and dried by a gentle heat, was suspended over some dry carbonate of potash, contained in a crystal phial covered with black silk, and placed in a dark closet, during the same space of time, underwent no visible change whatever.

Both these bits of silk were now taken out of the phials, and the gold was instantly restored to its metallic splendour by means of water and a solution of phosphorus in ether: a proof that the nitro-muriate of gold, in these bits of silk, suffered no change, during that space of time.

Exp. 5. Gold.

A piece of silk, which was dipped in the same solution of gold, and placed on a china plate exposed to the sunbeams, was kept moderately wet with water during the experiment: the first alteration observed was, that the yellow colour of the silk began to change to a faint green, succeeded by a purple tinge, which in about fifteen minutes exhibited some particles of reduced gold: soon after the threads of the silk, which acquired this purple tint, were gilded with gold; and, in about an hour, the whole of the silk was covered with a superb coat of reduced gold, exhibiting the texture of the silk in a beautiful manner.

The purple tinge, which attended the reduction verged in some parts on red, and in others on blue.

This experiment, which is very amusing, I have repeated times unnumbered: and when the sun is powerful, and the solution of gold properly prepared, and of due strength, the coat of reduced gold is so bright, and dazzling, as to distress the eye of the beholder.

I find, that the best way of preparing a solution of gold for this purpose is to separate the oxid of gold from its solution in nitro-muriatic acid by means of good ether.

The silk may be dipped in this solution, and when the ether evaporates, kept moderately wet with distilled water.

It was with this solution in ether that I first reduced gold in the fibres of silk, and it was by means of the same, I discovered, that water was a necessary condition in these reductions by light.

Exp. 6. Gold.

A bit of silk, which was dipped in a solution of nitro-muriate of gold in alcohol, and placed on a china saucer, exposed to the sunbeams, was kept wet with alcohol for the space of an hour; but no change even in the colour, which the solution of gold gave the silk, could be perceived.

The alcohol diffused the solution of gold unequally through the silk: the rays of the sun being intercepted, suspended the experiment: there was no sunshine on the following day: on the third day the silk was again exposed to the sunbeams, and kept wet with alcohol, which extracted a yellow tincture; a proof that the gold in the silk suffered no degree of reduction: the sun shone pretty strong for an hour, or more; and, at length, a faint tinge of purple, followed by some reduced gold, appeared.

The preceding experiments leave us no room to doubt, that the purple tinge and the small quantity of reduced gold, that appeared in this experiment, depended on water attracted from the air, or deposited in the silk by the alcohol during its evaporation.

Mr. Scheele reduced a solution of nitro-muriate of gold in water by exposing it in a phial for a fortnight to the rays of the sun⁠[23].

Mr. Lewis also made many experiments on the staining of marble, and other substances with metallic solutions, and light⁠[24].

Exp. 7. Silver.

A piece of silk, which was immersed in a solution of nitrate of silver in water, was dried by a gentle heat, and exposed to the light of the sun as much as possible in the window of a chambre from the 20th of July to the 20th of October.

In less than an hour the silk acquired a reddish brown colour: next day the colour became more intense, and gradually increased, till, on the third day, it bordered on black, which increased slowly: at length part of it became gray, and a few minute particles of reduced silver could be distinguished: the black tinge gradually disappeared, and the silk was of a reddish brown colour: the reduced silver had a gray cast.

The following experiments demonstrate, that the changes of colour, and the few particles of semireduced silver depended on water attracted from the atmosphere.

Exp. 8. Silver.

A bit of silk was immersed in part of the same solution of nitrate of silver in water, and dried in the dark: the silk retained its white colour: it was then suspended over dry carbonate of potash in a crystal phial, which was corked, and secured from the ingress of moisture by covering the cork with wax.

The phial was left in a dark closet 24 hours, that the salt might imbibe as much moisture as possible from the silk, and air.

The phial was then placed in a window, exposed, as much as possible, to the rays of the sun, from the 24th of July to the 20th of October: the silk scarcely suffered any visible change, except a very faint tinge of reddish brown; which was best seen by transmitted light; for the silk viewed by reflected light appeared nearly white: but that even this tinge, slight as it was, depended on a minute quantity of moisture, which the carbonate of potash was unable to extract, appears from the next experiment.

Exp. 9. Silver.

A piece of silk was dipped in a solution of fused nitrate of silver in alcohol, and carefully dried: it was then suspended over a quantity of concentrated sulphuric acid in a crystal phial, which was coated with a double fold of black paper, and gum arabic, reaching somewhat above the level of the acid, to prevent the light from acting on it: the phial was corked, and to exclude moisture more effectually, the cork was covered with wax.

The phial was now placed in a dark closet 24 hours, that the humidity of the enclosed silk and air might be attracted by the acid: it was then placed in a window, exposed as much as possible to the rays of the sun, from the 19th of July to the 26th of October: but the silk underwent no visible change whatever: its white colour remained pure, and unaltered.

The silk, at the end of this period, was taken out of the phial, and wetted with water: a solution of phosphorus in ether was then applied to it; and instantly the metallic splendour of the silver appeared.

Another bit of silk, which was immersed in a solution of nitrate of silver in water, and dried in the dark, was suspended over dry carbonate of potash in a phial, and placed in a dark closet about two months; the silk underwent no visible alteration whatever; its white colour remained pure.

I was desirous of knowing, if the nitrate of silver in this bit of silk suffered any change, that might render it incapable of reduction: I therefore took it out of the phial, and divided it into two parts: one of these was wetted with water, and the silver was instantly reduced by a solution of phosphorus in ether. The other part, which was suspended in a window, exposed to the air of the chamber, and rays of the sun, soon acquired a reddish brown colour.

Hence it is evident, that these changes of colour indicate partial reductions of the metal; and that they never happen without the presence of water.

Exp. 10. Silver.

A bit of silk, which was dipped in a solution of nitrate of silver in water, was placed on a China saucer, exposed to the rays of the sun, and kept moderately wet with water: in a few minutes the white colour of the silk was changed to a reddish brown, which by degrees became darker; and, in about three or four hours, though the solar rays were often languid, and intercepted, acquired a blackish gray colour, most of the reddish brown having disappeared: next day there was no sunshine; but towards the evening particles of reduced silver were visible on the side of the silk opposed to the light.

Exp. 11. Silver.

A bit of silk, which was immersed in a solution of nitrate of silver in alcohol, and exposed to the beams of the sun, as in the preceding experiment, was kept moderately wet with alcohol; but resisted the action of the light much longer than the preceding: however, in some time, specks, and lines, of a reddish brown, began to appear on some parts of the silk: next day, though there was no sunshine, the reddish brown colour increased a little: on the third day, it became more intense; but no black, or gray colour, or particle of reduced silver, could be observed.

Whoever compares this with the preceding experiments, can have no doubt that the changes of colour, which appeared in this experiment, depended on water attracted from the air, or deposited in the silk by the alcohol during its evaporation.

The experiments detailed in this Chapter, prove beyond the power of contradiction,

1. That water is essential to the reduction of metals by light: for these experiments demonstrate, that the reduction can not take place without water, and that it is always in proportion to the quantity of that fluid present.
2. That light does not reduce metals by giving them phlogiston, as Messrs. Macquer and Scheele supposed; for were this opinion true, light should reduce them without the aid of water, and as well with alcohol as with water.
3. Light does not reduce metals by fusing and expelling their oxygen, as the antiphlogistians imagine; for were this its mode of agency, the reduction should happen without the assistance of water, and as well with alcohol as with water.
4. Light is a combustible body; for it acts like hydrogen, phosphorus, sulphur, and charcoal, in the reduction of metals.

Since water then is essential to the reduction of metals by light, and since light does not reduce metals by giving them phlogiston, nor by fusing, and expelling their oxygen, it follows, that the water is either decomposed, or that it unites with metallic earths, and constitutes their phlogiston, or that it unites with, and separates their oxygenous principle: but as the two last suppositions are inadmissible, it is obvious that light reduces the metals by decomposing water.

After explaining the manner in which other combustible bodies reduce the metals by decomposing water; it will not be difficult to explain how light too produces the same effect.

But in order to facilitate the explanation, it must be premised,

1. That it is a law of attraction, that, when any body is deprived, to a certain degree, of another, for which it has a strong affinity, the attraction of the former for the latter is much increased.
2. That light has a strong attraction for oxygen, or the base of vital air.
3. That when oxygen is condensed, and fixed in any substance, it contains much less light, and caloric, than it does in the gazeous state.
4. That the oxygenous principle exists in water in this condensed state, and consequently has a strong attraction for light.

Therefore, when light reduces the metals, it attracts the oxygen of the water, while the hydrogen of the latter unites, in its nascent state, to the oxygen of the metal, and reduces it, forming at the same time a quantity of water equal to that decomposed.

Hence it follows that the light is oxygenated, and changed into vital air, while the metal is restored to its combustible state.