Sir H. Davy succeeded in decomposing the boracic acid, (which had till then been considered as undecompoundable,) by various methods. On exposing this acid to the Voltaic battery, the positive wire gave out oxygen, and on the negative wire was deposited a black substance, in appearance resembling charcoal. This was the basis of the acid, which Sir H. Davy has called _Boracium_, or _Boron_. The same substance was obtained in more considerable quantities, by exposing the acid to a great heat in an iron gun-barrel. A third method of decomposing the boracic acid consisted in burning potassium in contact with it in vacuo. The potassium attracts the oxygen from the acid, and leaves its basis in a separate state. The recomposition of this acid I shall show you, by burning some of its basis, which you see here, in a retort full of oxygen gas. The heat of a candle is all that is required for this combustion.-- EMILY. The light is astonishingly brilliant, and what beautiful sparks it throws out! MRS. B. The result of this combustion is the boracic acid, the nature of which, you see, is proved both by analytic and synthetic means. Its basis has not, it is true, a metallic appearance; but it makes very hard alloys with other metals. EMILY. But pray, Mrs. B., for what purpose is the boracic acid used in manufactures? MRS. B. Its principal use is in conjunction with soda, that is, in the state of _borat of soda_, which in the arts is commonly called borax. This salt has a peculiar power of dissolving metallic oxyds, and of promoting the fusion of substances capable of being melted; it is accordingly employed in various metallic arts; it is used, for example, to remove the oxyd from the surface of metals, and is often employed in the assaying of metallic ores. Let us now proceed to the FLUORIC ACID. This acid is obtained from a substance which is found frequently in mines, and particularly in those of Derbyshire, called _fluor_, a name which it acquired from the circumstance of its being used to render the ores of metals more fluid when heated. CAROLINE. Pray is not this the Derbyshire spar, of which so many ornaments are made? MRS. B. The same; but though it has long been employed for a variety of purposes, its nature was unknown until Scheele, the great Swedish chemist, discovered that it consisted of lime united with a peculiar acid, which obtained the name of _fluoric acid_. It is easily separated from the lime by the sulphuric acid, and unless condensed in water, ascends in the form of gas. A very peculiar property of this acid is its union with siliceous earths, which I have already mentioned. If the distillation of this acid is performed in glass vessels, they are corroded, and the siliceous part of the glass comes over, united with the gas; if water is then admitted, part of the silex is deposited, as you may observe in this jar. CAROLINE. I see white flakes forming on the surface of the water; is that silex? MRS. B.