Observing magnesia to lose such a remarkable proportion of its weight in the fire, my next attempts were directed to the investigation of this volatile part; and, among other experiments, the following seemed to throw some light upon it:-- Three ounces of magnesia were distilled in a glass retort and receiver, the fire being gradually increased until the magnesia was obscurely red hot. When all was cool, I found only five drams of a whitish water in the receiver, which had a faint smell of the spirit of hartshorn, gave a green colour to the juice of violets, and rendered the solutions of corrosive sublimate, and of silver, very slightly turbid. But it did not sensibly effervesce with acids. The magnesia, when taken out of the retort, weighed an ounce, three drams, and thirty grains, or had lost more than half of its weight. It still effervesced pretty briskly with acids, though not so strongly as before this operation. The fire should have been raised here to the degree requisite for the perfect calcination of magnesia. But, even from this imperfect experiment, it is evident, that, of the volatile parts contained in that powder, a small proportion only is water; the rest cannot, it seems, be retained in vessels, under a visible form. Chemists have often observed in their distillations that part of a body has vanished from their senses notwithstanding the utmost care to retain it; and they have always found, upon further inquiry, that subtle part to be air, which having been imprisoned in the body, under a solid form, was set free, and rendered fluid and elastic by the fire. We may therefore safely conclude, that the volatile matter lost in the calcination of magnesia, is mostly air; and hence the calcined magnesia does not emit air, or make an effervescence when mixed with acids. The water, from its properties, seems to contain a small portion of volatile alkali, which was probably formed from the earth, air and water, from some of these combined together; and perhaps also from a small quantity of inflammable matter, which adhered accidently to the magnesia. Whenever chemists meet with this salt, they are inclined to ascribe its origin to some animal or putrid vegetable substance; and this they have always done, when they obtained it from the calcareous earths, all of which afford a small quantity of it. There is, however, no doubt, that it can sometimes be produced independently of any such mixture, since many fresh vegetables, and tartar, afford a considerable quantity of it. And how can it, in the present instance, be supposed, that any animal or vegetable matter adhered to the magnesia, while it was dissolved by an acid, separated from this by an alkali, and washed with so much water?