Imagining that I perceived some resemblance between the properties of _magnesia_ and those of alkalis, I was led to try what change this substance would suffer from the addition of quick-lime, which alters in such a peculiar manner the alkaline salts. Twenty seven grains of _magnesia_ in fine powder were mixed with eighteen ounces of lime-water in a flask, which was corked close and shaken frequently for four days. During this time, I frequently dipp'd into it little bits of paper, which were coloured with the juice of violets; and these became green as soon as they touched the water, until the fourth day, when their colour did not seem to be altered. The water being now poured off, was intirely insipid, and agreed in every chemical trial with pure water. The powder, after being perfectly well dried, weighed thirty seven grains. It did not dissolve intirely in spirit of vitriol; but, after a brisk effervescence, part of it subsided in the same manner as the calcarious earths, when mixed with this acid. When I first tried this experiment, I was at the trouble of digesting the mixture in the heat of boiling water, and did not then know that it would succeed in the heat of the air. But Dr. _Alston_, who has obliged the world with many curious and useful discoveries on the subject of quick-lime, having had occasion to repeat it, I learned from him that heat is not necessary; and he has moreover added an useful purpose to which this property of _magnesia_ may be applied; I mean the sweetening of water at sea, with which lime may have been mixed to prevent its putrefaction. That part of the dried powder which does not dissolve in spirit of vitriol, consists of the lime separated from the water. Quick-lime itself is also rendered mild by _magnesia_, if these two are well rubbed together and infused with a small quantity of water. By the following experiments, I proposed to know whether this substance could be reduced to a quick-lime. An ounce of _magnesia_ was exposed in a crucible for about an hour to such a heat as is sufficient to melt copper. When taken out, it weighed three drams and one scruple, or had lost 7/12 of its former weight. I repeated, with the _magnesia_ prepared in this manner, most of those experiments I had already made upon it before calcination, and the result was as follows. It dissolves in all the acids, and with these composes salts exactly similar to those described in the first set of experiments: but what is particularly to be remarked, it is dissolved without any the least degree of effervescence. It slowly precipitates the corrosive sublimate of mercury in the form of a black powder. It separates the volatile alkali in salt ammoniac from the acid, when it is mixed with a warm solution of that salt. But it does not separate an acid from a calcarious earth, nor does it induce the least change upon lime-water.