That is the most important metallic salt formed by sulphuric acid, and the only one that we shall here notice. It is of great use in the arts; and, in medicine, it affords a very valuable tonic: it is of this salt that most of those preparations called _steel medicines_ are composed. CAROLINE. But does any carbon enter into these compositions to form steel? MRS. B. Not an atom: they are, therefore, very improperly called steel: but it is the vulgar appellation, and medical men themselves often comply with the general custom. Sulphat of iron may be prepared, as you have seen, by dissolving iron in sulphuric acid; but it is generally obtained from the natural production called _Pyrites_, which being a sulphuret of iron, requires only exposure to the atmosphere to be oxydated, in order to form the salt; this, therefore, is much the most easy way of procuring it on a large scale. EMILY. I am surprised to find that both acids and compound salts are generally obtained from their various combinations, rather than from the immediate union of their ingredients. MRS. B. Were the simple bodies always at hand, their combinations would naturally be the most convenient method of forming compounds; but you must consider that, in most instances, there is great difficulty and expense in obtaining the simple ingredients from their combinations; it is, therefore, often more expedient to procure compounds from the decomposition of other compounds. But, to return to the sulphat of iron. --There is a certain vegetable acid called _Gallic acid_, which has the remarkable property of precipitating this salt black--I shall pour a few drops of the gallic acid into this solution of sulphat of iron-- CAROLINE. It is become as black as ink! MRS. B. And it is ink in reality. Common writing ink is a precipitate of sulphat of iron by gallic acid; the black colour is owing to the formation of gallat of iron, which being insoluble, remains suspended in the fluid. This acid has also the property of altering the colour of iron in its metallic state. You may frequently see its effect on the blade of a knife, that has been used to cut certain kinds of fruits. CAROLINE. True; and that is, perhaps, the reason that a silver knife is preferred to cut fruits; the gallic acid, I suppose, does not act upon silver. --Is this acid found in all fruits? MRS. B. It is contained, more or less, in the rind of most fruits and roots, especially the radish, which, if scraped with a steel or iron knife, has its bright red colour changed to a deep purple, the knife being at the same time blackened. But the vegetable substance in which the gallic acid most abounds is _nutgall_, a kind of excrescence that grows on oaks, and from which the acid is commonly obtained for its various purposes. MRS. B. We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating of phosphorus, you have seen how these acids may be obtained from it by combustion? EMILY.