OBSERVATIONS.
A low cucurbit is recommended for this operation, to the intent that the particles of the Nitrous Acid, which are ponderous, may the more easily be carried up and pass over into the receiver. For the same reason the cucurbit is directed to be almost wholly covered with sand, lest otherwise the acid vapours should be condensed about that part of the cucurbit, which, being out of the sand, would be much cooler than that which is encompassed therewith, and from thence should fall back again to the bottom; by which means the distillation would certainly be retarded, and the vessel probably be broken.
Notwithstanding these precautions the vessels are liable to break in such distillations; especially when they contain a great deal of liquor. With a view, therefore, to prevent this accident, we ordered that the whole quantity of the solution of Silver to be distilled should not be put at once into the alembic. The little bit of tallow, added towards the end of the operation, is intended to hinder the metal from adhering closely to the vessel, as it would otherwise do, when all the moisture is dissipated.
The Soap and Fixed Alkali mixed with the Silver to flux it, after its separation from the aqua fortis in this way, serve to absorb such of the most fixed particles of the Acid as may still remain united with the metal.
If the distillation be stopped when part of the phlegm is drawn off, and the liquor be then suffered to cool, many crystals will shoot therein, which are a Neutral Salt constituted of the Nitrous Acid and Silver. If the distillation be carried further, and stopped when near its conclusion, the liquor being then suffered to cool will wholly coagulate into a blackish mass called the Infernal Stone.
This way of separating Silver from its solvent is attended with the advantage of saving all the aqua fortis, which is excellent, and fit to be employed in other operations.
PROCESS VI.
To separate Silver from the Nitrous Acid by Precipitation. Luna Cornea. Luna Cornea reduced.
Into your solution of Silver pour about a fourth part in weight of Spirit of Salt, solution of Sea-salt, or solution of Sal Ammoniac. The liquor will instantly become turbid and milky. Add twice or thrice its weight of fair water, and let it stand some hours to settle. It will deposite a white powder. Decant the clear liquor, and on the precipitate pour fresh aqua fortis, or Spirit of Salt, and warm the whole on a sand-bath with a gentle heat for some time. Pour off this second liquor, and boil your precipitate in pure water, shifting it several times, till the precipitate and the water be both quite insipid. Filter the whole, and dry the precipitate, which will be a Luna Cornea, and must be reduced in the following manner.