That the air which constitutes the atmosphere in which we live has _weight_, and that it is _elastic_, or consists of a compressible and dilatable fluid, were some of the earliest discoveries that were made after the dawning of philosophy in this western part of the world. That elastic fluids, differing essentially from the air of the atmosphere, but agreeing with it in the properties of weight, elasticity, and transparency, might be generated from solid substances, was discovered by Mr. Boyle, though two remarkable kinds of factitious air, at least the effects of them, had been known long before to all miners. One of these is heavier than common air. It lies at the bottom of pits, extinguishes candles, and kills animals that breathe it, on which account it had obtained the name of the _choke damp_. The other is lighter than common air, taking its place near the roofs of subterraneous places, and because it is liable to take fire, and explode, like gunpowder, it had been called the _fire damp_. The word _damp_ signifies _vapour_ or _exhalation_ in the German and Saxon language. Though the former of these kinds of air had been known to be noxious, the latter I believe had not been discovered to be so, having always been found in its natural state, so much diluted with common air, as to be breathed with safety. Air of the former kind, besides having been discovered in various caverns, particularly the _grotta del Cane_ in Italy, had also been observed on the surface of fermenting liquors, and had been called _gas_ (which is the same with _geist_, or _spirit_) by Van Helmont, and other German chymists; but afterwards it obtained the name of _fixed air_, especially after it had been discovered by Dr. Black of Edinburgh to exist, in a fixed state, in alkaline salts, chalk, and other calcareous substances. This excellent philosopher discovered that it is the presence of the fixed air in these substances that renders them _mild_, and that when they are deprived of it, by the force of fire, or any other process, they are in that state which had been called _caustic_, from their corroding or burning animal and vegetable substances. Fixed air had been discovered by Dr. Macbride of Dublin, after an observation of Sir John Pringle's, which led to it, to be in a considerable degree antiseptic; and since it is extracted in great plenty from fermenting vegetables, he had recommended the use of _wort_ (that is an infusion of malt in water) as what would probably give relief in the sea-scurvy, which is said to be a putrid disease. Dr. Brownrigg had also discovered that the same species of air is contained in great quantities in the water of the Pyrmont spring at Spa in Germany, and in other mineral waters, which have what is called an _acidulous_ taste, and that their peculiar flavour, briskness, and medicinal virtues, are derived from this ingredient.