Lastly, when a dram of it is digested with an ounce of water in a bottle for some hours, it does not make any the least change in the water. The _magnesia_, when dried, is found to have gained ten grains; but it neither effervesces with acids, nor does it sensibly affect lime-water. 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 the half of its weight. It still effervesced pretty briskly with acids, tho' 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 subtile 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.