COMBINATIONS OF EARTHS. 
All of these classes have been mentioned already, and 
instances given of minerals belonging to each except 
the fourth, to which belong only the zircon and the 
byacinth. 
The most beautiful earthy combinations are those 
which consist chiefly of silica and alumina. To that 
class belong almost all the gems. The combinations of 
these two earths are also the most useful; for they con 
stitute the basis of porcelain, stoneware, glass pots, 
and all the different earthen utensils. 
9. During the fusion of several mixtures or combi 
nations of earths, especially those which are found na 
tive, the mixture very often frothes considerably, and 
emits a great number of air bubbles. The emission of 
these air bubbles can only be referred to one or other of 
three causes. Either a quantity of water leaves the 
mixture in the form of steam, or the metallic oxides, 
which are almost always present, are reduced, and al 
low their oxygen to escape in the form of gas ; or, 
lastly, the earths themselves undergo some change, and 
let go some one or other of their component parts in 
the form of gas. It is probable that the phenomenon is 
partly owing to all these causes ; for these earthy mix 
tures are much lighter after having been exposed to the 
action of the fire than they were before. Thus a mix 
ture of equal parts of silica and magnesia, after being 
heated to 134° Wedgewood, lost o.135 of its weight; 
and a mixture of equal parts of alumina and barytes 
lost in the same situation o.275 of its weight *. Now 
so great a loss as this cannot be ascribed to any other 
cause than the evaporation of water. 
* Morveau, Jour. de l'Ecole Polytechnique, I. iii. 306. 
233 
Chap. I. 
Earthsoften 
frothe du 
ring fusion.