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some of the Substances which accompany it , & c. 
cannot be dissolved by the acids : this substance, however, 
which is silica, has been in solution in the alkali. But, if a 
greater proportion of alumina be present, none of this flocculent 
precipitate appears; hence it is evident, that alumina must 
determine its solution. Its easy solubility, in the latter case, 
cannot depend upon the division of the particles of the silica in 
the stone; for, in the first place, after being fused with potash, 
the tenuity of the particles of every stone must be nearly the 
same; and, in the next place, I have not observed, that any 
earth, except alumina, can promote the chemical solution of the 
silica, though they must all occasion its mechanical division. 
As to the affinity of alumina for magnesia, it is by much 
the most powerful of all those which any of the earths have 
for each other. I attempted to precipitate magnesia from 
muriatic acid, by ammonia, even in excess ; but found that the 
whole muriate of magnesia had not been decomposed, and that 
a triple salt, or an ammoniacal muriate of magnesia,* had been 
formed. I then poured an excess of ammonia into a solution 
of muriate of magnesia, mixed with a large proportion of a 
solution of muriate of alumina. All the earth was precipitated ; 
and nothing remained in solution, except muriate of ammonia* 
The liquor was then filtered, and the precipitate washed and 
dried. I dissolved it in muriatic acid, and boiled it with a great 
excess of potash. Some alumina was taken up, but by no means 
all the quantity that had been used. The precipitate which had 
resisted the action of potash, was again dissolved in muriatic 
acid, and precipitated by carbonate of potash. The carbonate of 
magnesia was held in solution by the excess of carbonic acid; 
and, by using potash and- carbonic acid alternately, (the first , to 
* This salt is well known in chemistry. 
