JOSEPH BLACK 93 



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 calcareous earth, nor does it introduce 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 follo\ving 

 seemed to throw some light upon it: — 



Three ounces of magnesia were distilled in a glass retort and re- 

 ceiver, 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 eflFervesce 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 eflFervesced 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 



