Arseniate of Lead

This new and at present very rare substance, but lately discovered in Cornwall, is a token of our possessing in our island some substances that are not found elsewhere. I am obliged to the Rev. William Gregor (who has been mentioned as the first person who analysed the Menachanite, tab. 277.) for ascertaining what this substance is, and also for the delicate specimen figured. In colour and form of i he crystal it somewhat resembles Phosphate of Lead,—see tab. 84,— which, however, is seldom so brown as this, and is less transparent: besides, the crystals of Arseniate have some characters which I do not know to occur in the Phosphate of Lead, such as the fasciculated structure found in most of the crystals, even where the two ends are apparently regular; also a considerable degree of lustre, transparency, and neatness at the terminal edges: besides, it is considerably softer than Phosphate of Lead, and rather harder than Sulphate of Lead. The crystals are scattered upon a gangue of white Quartz.

With the specimen sent by Mr. Gregor, he favoured me with the following account, which, as it is likely to be very useful to the science, I consider it proper to publish in nearly his own words.

“The mineral which I lately sent to you was raised in a cress lode of Huel Unity in the parish of Gwennap in Cornwall. At what depth or under what circumstances I am unable to discover. A very small quantity only of it has been raised. I shall briefly mention some of the reasons which induced me to pronounce this fossil to be an Arseniate of Lead. A small fragment of it exposed to the flame of a blowpipe in a golden spoon loses its yellowish brown colour, and becomes white. Urged still further it melts into a brownish mass, and continues in this fused state without decomposition. If this mass be detached from the spoon and placed upon charcoal, and if the flame be directed upon it, a decomposition rapidly takes place, and fumes, evidently arsenical, are extricated. Globules of a fused metal are left upon the charcoal, which is marked with that yellowish hue which indicates the presence of Lead. This fused metal is malleable: it is soluble in Nitric Acid, and forms with it a crystallized salt, the solution of which is rendered turbid by a drop of liquid Sulphate of Soda, and a heavy white matter subsides. A piece of the mineral does not appear to be acted upon by Nitric Acid in the cold. By long digestion, however, in a sand-bath, the solution is effected. The transparency of this solution is not disturbed by a drop of liquid Muriate of Barytes. Some crystals, however, appeared in small quantity, after some hours, at the bottom of the glass. A drop of liquid Nitrate of Silver caused the nitric solution to assume an opalescence, scarcely however perceptible. The purest crystals were selected; but it is impossible to free them perfectly from all the stony matter which adheres to them. With a view to this extraneous increment, I weighed 26.2 grains, and exposed the powder in a platinum crucible to a very low heat, in which the bottom of the vessel was scarcely red, for half an hour. I proceeded thus cautiously, because the Arsenic Acid is decomposed under certain circumstances, and it was a greater object with me to ascertain the quantum of acid than of the water of crystallization. The matter lost 0.48 of a grain, I have reason to think that all the water of crystallization was not expelled. It was now transferred to a silver crucible, and a ley containing about 20 grains of Potash was gradually digested with it, to dryness, and the crucible was at last exposed for a few minutes on an open fire. All the soluble part was elixated by distilled water, and the solid matter left behind weighed 19.50 grains. I found that the alkaline ley held a small portion of the mineral in solution; this was previously separated: it consisted of Oxide of Lead, Silica, and very slight traces of Alumina and Iron, After being neutralized with Nitric Acid, liquid Nitrate of Lead was dropped into it, as long as any precipitate was produced. The precipitated white matter weighed 14 grains, which, according to the calculation of Mr. Chenevix, indicates 4.62 of Arsenic Acid. In order to be certified that it contained Arsenic Acid, I decomposed it by means of half its weight of Sulphuric Acid, I obtained a dry granulated matter which attracted the moisture of the atmosphere. It had acid properties. Dissolved in water, it precipitated Oxide of Titanium from a solution of its Sulphate. Some of this acid was exposed to the flame by the blowpipe in a golden spoon: it bore ignition, and appeared to be a dry pulverulent matter; but being heated on charcoal it was speedily dissipated in copious arsenical vapours. Some of the Acid was neutralized by Carbonate of Soda. A solution of this Salt, mixed with liquid Nitrate of Silver, produced a copious precipitate of a brick-colour. As I thought it possible that all the acid had not been extracted, I boiled the 19.50 grains with a fresh ley of Potash, Some of the mineral was again dissolved, but not the smallest quantity of Arsenic Acid was obtained. The remaining matter was dissolved in Nitric Acid, except a small portion of Quartz. The matter, also, which had been separated from the alkaline ley, was dissolved in the same acid. The solution was assayed by a drop of Muriatic Acid, which caused the precipitation of a white matter; but it was redissolved on shaking the fluid,—a circumstance which proves the absence of Silver. The Lead was separated by liquid Sulphate of Soda, and added together, the Oxide of which, according to calculation, amounted to 17.179. Ammonia afterwards precipitated a greenish white matter, a portion of which a further addition of Ammonia redissolved. The former proved to be Oxide of Iron = 0.17, and the latter Oxide of Copper which amounted to about 0.1. The Silica which remained undissolved by Nitric Acid, and which was separated from the Potash , amounted to 0.815. The Alumina was in too small a quantity to be estimated. This, and the Silica, and probably the minute portion of Copper also, proceeded, I believe, from the gangue of this fossil* There is a deficiency of 2.836, which is to be attributed in part, I think, to a relent of water of crystallization, in addition to the loss which is unavoidable in such experiments, more especially when small quantities are operated upon, and a repetition of experiments is precluded from the scarcity of the mineral.”

Since the above was written, Mr. Gregor, having been enabled to extend his experiments, has informed me that he suspects the presence of the Muriatic as well as the Arsenic Acid in this substance, and that he intends presenting the Royal Society with an account of the results of his more extended experiments.