394 
Book II. 
Magnesia 
and alumi 
na, 
Silica. 
ANALYSIS OF 
occasions a white precipitate in water, containing a very 
minute proportion of this earth. To render this test 
decisive, the following precautions are necessary: 1. The 
mineral acids, if any be present, must be previously 
saturated with an alkali. 2. Barytes, if any be present, 
must be previously removed by means of sulphuric acid. 
3. Oxalic acid precipitates magnesia but very slowly, 
whereas it precipitates lime instantly. 
XIV. Magnesia and alumina. The presence of these 
earths is ascertained by the following tests : 1. Pure 
ammonia precipitates them both, and no other earth, 
provided the carbonic acid has been previously separated 
by a mineral alkali and boiling. 2. Lime-water preci 
pitates only these two earths, provided the carbonic 
acid be previously removed, and the sulphuric acid also, 
by means of nitrat of barytes. 
The alumina may be separated from the magnesia 
after both have been precipitated together, either by 
boiling the precipitate in pure potass, which dissolves 
the alumina and leaves the magnesia ; or the precipitate 
may be dissolved in muriatic acid precipitated by an al 
kaline carbonat, dried in the temperature of a 100°, 
and then exposed to the action of diluted muriatic acid, 
which dissolves the magnesia without touching the alu 
mina. 
XV. Silica may be ascertained by evaporating a por 
tion of the water to dryness, and redissolving the préci 
pitate in muriatic acid. The silica remains behind un 
dissolved. 
Such is the method of detecting the different sub 
stances commonly found in waters. But as these different 
substances are almost always combined together, so as to 
constitute particular salts, it is not sufficient to know in