j. Carbonate of soda, and oxalate of potash, and solutions of magnesia, clay, copper, iron, and zinc, either had no effects, or extremely slight ones.

k. Solution of sulphate of silver produced a white curd-like precipitate. 9.35 grains of this salt (the weight of the insoluble matter being deducted) afforded 1,05 grains of slightly melted muriate, or chloride, of silver. This precipitate was equally produced after the salt had been made strongly red hot, so that it was not owing to a portion of sal ammoniac.

1. Tartaric acid, and muriate of platinum, occasioned the precipitates in its solution which indicate potash.

m. Nitrate of lime did not form any immediate precipitate in a dilute solution of it; but in a short time, numerous minute prismatic crystals of hydrate of sulphate of lime were generated.

n. Nitrate of barytes poured into a solution containing 9,8 grains of this salt afforded a precipitate, which after being ignited weighed 12,3 grains. The filtered solution crystallized entirely into nitrate of potash mixed with a few rhomboides of nitrate of soda.

o. Some of this salt finely pulverized was treated with alcohol. This alcohol on exhaling left a number of minute cubic crystals, which proved, by the test of nitric acid, to be muriate of soda. Prussiate of soda-and-iron caused a red precipitate of prussiate of copper in this alcoholic solution.

p. The solution of this salt afforded, by crystallization, sulphate of potash in its usual forms, and some prismatic crystals of hydrate of sulphate of soda.

q. To discover what had occasioned the precipitate with galls, (i) since copper has not this quality, a portion of this

salt, which had been recovered by evaporation from a filtered solution of it, was made red hot in a platina crucible. On extraction of the saline part by water, a very small quantity of a black powder was obtained. Ammonia dissolved only part of it, which was copper. The rest being digested with muriatic acid, and prussiate of soda-and-iron added, a fine Prussian blue was formed.

r. From several of the foregoing experiments, it appeared that no sensible quantity of any of the mineral acids, besides the sulphuric and muriatic, existed in combination with alkali in this volcanic salt. But Mr. TENNANT, whose many and highly important discoveries have so greatly contributed to the progress of chemical science, having detected disengaged boracic acid amongst the volcanic productions of the Lipari islands, and suggested that it might be a more general product of volcanoes than had been suspected,* it became important to ascertain whether the presence of any in this salt proved Vesuvius likewise to be a source of this acid. Alcohol heated on a portion of it in fine powder, and then burned on it, did not however shew the least green hue in its flame.

s. To ascertain the proportions of the ingredients of this saline substance, the following experiments were made:

10 grains of sulphate of potash of the shops were dissolved in 200 grains of water, and an excess of muriate of platina added. The precipitate edulcorated with 100 grains of water, and dried on a water bath, weighed 24,1 grains.

10 grains of the saline part of the native salt, treated precisely in every respect in the same way, afforded 17,2 grains of precipitated muriate of platina-and-potash.

* Trans. of the Geolog, Soc.

If 24,1 grains of this precipitate correspond to 10 grains of sulphate of potash, 17,2 grains of it correspond to 7,14 grains of this salt.

It has been seen (n) that 10 grains of the saline part of this volcanic salt would have afforded 12,55 grains of sulphate of barytes.

But 7,14 grains of sulphate of potash form only 9,42 grains of sulphate of barytes,* and therefore the remaining 3,13 grains of sulphate of barytes would be produced by the sulphate of soda, and correspond to 1,86 grains of it in an arid state, or uncombined with ice.†

10 grains of the saline part of this native salt would have produced 1,12 grains of ignited muriate of silver (k). By accurate experiments 241 grains of ignited muriate of silver have been found to correspond to 100 grains of ignited muriate of soda.‡

Consequently the soluble portion of the present Vesuvian salt consists of

t. The insoluble sandy residue (g) having been thoroughly

• Dr. MARCET on Dropsical Fluids.

+ Prof. KLAPROTH's Essays, Vol. I. p. 282.

Dr. HENRY, Phil. Trans. 1810.

edulcorated, dilute nitric acid was put to it. A green solution formed without any effervescence. Acetate of barytes scarcely rendered this solution turbid; but nitrate of silver produced a copious curd-like precipitate, and iron abundantly threw down copper from it. The green grains enclosed in this native sulphate of potash, appear, therefore, to be a submuriate of copper, of the same species as that of the green sands of Peru and Chili.

Muriatic acid dissolved the yellow ochraceous powder, and prussiate of soda-and-iron produced Prussian blue. I am inclined to believe this yellow powder to be a submuriate of iron, but its small quantity, and the admixture of the submuriate of copper, were impediments to entirely satisfactory results. Such a submuriate of iron, though, if I mistake not, overlooked by chemists, exists, for the precipitate which oxygen occasions in solution of green muriate of iron, contains marine acid.

Possibly this yellow powder, and the crystals of speculary iron which exist in this Vesuvian salt, have been produced by a natural sublimation of muriate of iron, similar to that of the experiment of the Duke d'AYEN, recorded by MACQUER,* and which was known long before to Mr. BOYLE and Dr. LEWIS.†

This Vesuvian salt, considered in its totality, has presented no less than nine distinct species of matters, and a more rigorous investigation, than I was willing to bestow on it, would probably add to their number.

July 3, 1813.

• Dict. de Chemie, Art. Fer.

+ A course of practical chemistry by WILLIAM LEWIS, 1746, page 63, note f.

XXXI. Some Experiments and Observations on the Substances produced in different chemical Processes on Fluor Spar. By Sir Humphry Davy, LL.D. F. R. S. V. P. R. I.

Read July 8, 1813.

In the Bakerian Lecture, for 1808, I have given an account of an experiment on the combustion of potassium in silicated fluoric acid gas, in which the gas was absorbed, and a fawn coloured substance formed, which effervesced with water, and left, after its action on that fluid, a residuum which burnt when heated in oxygen, reproducing silicated fluoric acid gas; and I concluded from the phenomena, that the acid gas was decomposed in the process, that oxygen was probably separated from it by the potassium, and that the combustible substance was a compound of the siliceous and fluoric bases.

The experiment of burning potassium in silicated fluoric acid gas was made likewise by M. M. GAY LUSSAC and THENARD, before I published any account of my researches on this phenomenon. It was indeed one of the most obvious арplications of potassium, and it occurred, to many others, as well as to myself, that it might be made, immediately after I discovered that metal.

M. M. GAY LUSSAC and THENARD drew the same conclusions as I did, namely, that the acid gas was probably decomposed during the action of potassium on silicated fluoric acid;