William Gregory to Faraday   7 February 1843¹

King's College 7th Feby | 1843

My dear Sir

I have to thank you, which I do most sincerely, for a copy of your Chemical Manipulation², which I have often of late sought for in vain, having given my old copy³ to a friend some years since. I need not say how much I value it, as every working Chemist must have the same high opinion of it.

I have lately proposed a new method of purifying silver, or rather of obtaining it as metal or oxide from the chloride⁴. I find that the moist chloride bruised in the dish as much as possible during its washing by decantation, is rapidly converted into oxide by boiling with a solution of caustic potash, of Sp.G. 1.25 to 1.3. The oxide thus obtained is pure black and very dense. It is sometimes necessary to grind the moist powder (drained first) in a mortar, after 5 or 10 minutes boiling, and then to return it into the liquid and boil for 5 minutes more. Should it, after this, not be entirely soluble in dilute Nitric Acid, it is to be dried and finely ground, which is easily done, and after this, another boiling completes the operation. With small quantities, however, of chloride, if carefully bruised, the first boiling generally succeeds.

It is obvious that the oxide, when pure, yields pure silver by simple heating. But even if it still contain, say 1/10 of its weight of unchanged chloride, the whole silver is readily obtained by heating the mass with a little borax and salt of tartar without the risk of failure which attends the reduction of the pure chloride, either from insufficient heat, or from a too intense heat, as often happens, causing the alkali to corrode the crucible. I find it much easier to obtain pure silver by this process than by any other, for when precipitated by copper, it seems to me generally to contain a trace of that metal, and is almost sure to contain some of the impurities of the copper. At all events it is seldom quite pure. The process is so easy that I dissolved half a crown, and obtained, not only the pure oxide but the fused metal quite pure, within two hours. Both the chloride and the oxide, especially the latter, are very quickly washed by decantation. The only precaution is, to make the latter washings of the oxide with cold water, as when nearly pure, it has a curious tendency to rise in hot water.

This process is admirably adapted for obtaining quickly an absolutely pure Nitrate from common silver. And here also the presence of undecomposed chloride is not injurious, as it is left undissolved by the acid.

The whole process, including the reduction of the silver by the heat of the spirit lamp, from the oxide is well adapted for the lecture table. The change from the white chloride to the black oxide is very striking.

Finally, we obtain a pure and beautiful oxide, in a new form, quite distinct from that of the oxide precipitated from the Nitrate. I have no doubt that it will meet with many applications. The new oxide dissolves instantaneously in very weak Nitric Acid. It is to be noted that if the chloride have been once dried it is decomposed with great difficulty, even by long boiling with potash[.] Still, if first very finely powdered, and, if necessary, again dried and pulverized or levigated, it may be decomposed. I should add that the new form of oxide seems to have the same composition as the common one. It dissolves without effervescence in dilute Nitric Acid.

Yours very truly | William Gregory.