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.
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.