Easy directions for impregnating water with fixed air I have published in a small pamphlet, designed originally for the use of seamen in long voyages, on the presumption that it might be of use for preventing or curing the sea scurvy, equally with wort, which was recommended by Dr. Macbride for this purpose, on no other account than its property of generating fixed air, by its fermentation in the stomach. Water thus impregnated with fixed air readily dissolves iron, as Mr. Lane has discovered; so that if a quantity of iron filings be put to it, it presently becomes a strong chalybeate, and of the mildest and most agreeable kind. I have recommended the use of _chalk_ and oil of vitriol as the cheapest, and, upon the whole, the best materials for this purpose. But some persons prefer _pearl ashes_, _pounded marble_, or other calcareous or _alkaline substances_; and perhaps with reason. My own experience has not been sufficient to enable me to decide in this case. Whereas some persons had suspected that a quantity of the oil of vitriol was rendered volatile by this process, I examined it, by all the chemical methods that are in use; but could not find that water thus impregnated contained the least perceivable quantity of that acid. Mr. Hey, indeed, who assisted me in this examination, found that distilled water, impregnated with fixed air, did not mix so readily with soap as the distilled water itself; but this was also the case when the fixed air had passed through a long glass tube filled with alkaline salts, which, it may be supposed, would have imbibed any of the oil of vitriol that might have been contained in that air[2]. Fixed air itself may be said to be of the nature of an acid, though of a weak and peculiar sort.----Mr. Bergman of Upsal, who honoured me with a letter upon the subject, calls it the _aerial acid_, and, among other experiments to prove it to be an acid, he says that it changes the blue juice of tournesole into red. This Mr. Hey found to be true, and he moreover discovered that when water tinged blue with the juice of tournesole, and then red with fixed air, has been exposed to the open air, it recovers its blue colour again. The heat of boiling water will expel all the fixed air, if a phial containing the impregnated water be held in it; but it will often require above half an hour to do it completely. Dr. Percival, who is particularly attentive to every improvement in the medical art, and who has thought so well of this impregnation as to prescribe it in several cases, informs me that it seems to be much stronger, and sparkles more, like the true Pyrmont water, after it has been kept some time. This circumstance, however, shews that, in time, the fixed air is more easily disengaged from the water; and though, in this state, it may affect the taste more sensibly, it cannot be of so much use in the stomach and bowels, as when the air is more firmly retained by the water.