BY HENKY G. STOKES, F.G.S. 
15 
the best-known instance of the hydrated oxide we have the so- 
called mineral bauxite. As examples again of alumina in a 
combined state we have the mineral spinel, of which again 
there are several varieties, which is the oxide of aluminium 
combined with the oxide of magnesium ; and kryolite, the 
fluoride of aluminium and sodium. By far the best illustration 
of this last mode of occurrence is, however, the hydrous silicate 
of alumina-clay, which enters so largely into the composition of 
clayey-slates, 'shales, i^nd earths, and of this the Cleveland 
mineral is an example. The hydrous silicate of alumina, or 
clay, when occurring in an approximately pure condition, is 
termed kaolin ; usually, however, it exists in a more or less im- 
pure state, the impurity consisting of sand, carbonate of lime, 
oxides of iron, and the alkalies ; whilst in many cases more or 
less carbonaceous matter is present. 
Passing in review these different combinations under 
which alumina is met with, as possible sources of the metal 
aluminium, we may at once eliminate the clays and kaolins, 
since it has nowdiere been found practicable to utilise hydrous 
silicate of alumina for its manufacture, owing to the great diffi- 
culty and consequent expense of liberating the alumina from 
the silica with which it is chemically combined. Incidentally, 
I too, it may be here pointed out that in published analyses of 
I clays giving alumina as occurring in a certain percentage, this 
should be regarded as being chemically combined with more or 
less of the silica which is represented in the same tabulation, 
since as above stated alumina does not exist in a free state in 
them. Again we may pass over the different gems illustrating 
the occurrence of alumina, either free or combined, already 
mentioned. In short, aluminium is at present derived from t-wo 
sources only — namely, the above-mentioned minerals, kryolite 
and bauxite, neither of which, as far as can be ascertained, are 
yet known to occur in these colonies. 
In the case of the former of these, i.e,, kryolite, the double 
ffuoride of aluminium and sodium, which was until quite 
recently extensively employed as a source of aluminium, it may 
be pointed out that the impurities which it contains (princi- 
pally iron and silicon compounds), and a limited supply, have 
