us 
THE GEOLOUIST. 
salt called chloride of alumiumm, then we can, by means of a metal having 
a powerful affinity for the chlorine, succeed in separating the aluminium 
completelj^ The first process was to take aluminium and mix that with 
charcoal, and then to make the mixture into small pellets. Now, charcoal 
at a high temperature has an affinity for oxj'-gen ; and if we expose that 
mixture of alumina and charcoal at a certain degree of heat to the action 
of chlorine, we then get chloride of aluminium. "We have then two 
affinities coming into play — the affinity of the oxygen for the carbon, and 
of the chlorine for the aluminium, and by means of this twofold force we 
form this body, having a yellow colour, called chloride of aluminium. 
'^A'e then place that chloride of aluminium in a glass vessel, which was 
the way it was formerly prepared, and expel the air by means of a hydro- 
gen gas, and then introduce the vapour of sodium by^ a new application of 
heat to the exterior, in contact with the vapour of the chloride of alu- 
minium. The result is the formation of chloride of sodium or common 
salt, and the separation of the metal. There is one other source of alumi- 
nium from a curious mineral called cryolite — one of the highest possible 
interest. Here is a specimen from Greeidand. This consists of the ele- 
ments fluorine, aluminium, and sodium. Well, to any mineralogist and 
chemist who reflected upon its composition, it would naturally occur that, 
if we could bring that substance into contact with sodium at a high tempe- 
rature, we should produce aluminium. The thought occurred here some 
time ago, and the very first specimen produced ^^ as produced in this place. 
That specimen was shown some time ago at the Royal Institution. 
Alumina is a compound of oxygen and aluminium. It contains, in round 
numbers, 53 S per cent, of aluminium, and, consequently, 46*7 per cent, of 
oxygen, and its chemical symbol is Alg O3. When dry, it forms a white, 
tasteless powder, insipid and insoluble. This powder is fusible only at 
the very highest temperatures we can command ; in fact, it is singularly 
infusible. In that state it is entirely amorphous. It has the property of 
combining with water in several proportions, producing a plastic clay-like 
mass. Compounds of this kind occur in nature. One hydrate— that is to 
say, one water-compound of alumina — occurs beautifully crystallized. 
That is the well-known diaspore. Then we have a non-crystalline variety 
in the form of Gibbsite. This diaspore is a compound of one equivalent 
of alumina and one of water. Gibbsite, of which you have a specimen 
before you, is a white, amorphous, or non-crystalline body, and is a com- 
pound of one equivalent of alumina and three of water. Alumina crys- 
tallizes magnificently, producing that glorious mineral (excuse the emphatic 
expression) called corundum, which, when blue, is known to you as sap- 
phire, when red as ruby, when yellow as oriental topaz, and when green 
as emerald. We have here a specimen of corundum which is coloured, 
forming these various minerals respectively. We shall speak of the mode 
of formation presently. 
Alumina acts in the twofold capacity of base and acid — that is to say, 
it unites with acids forming definite salts, and it also acts as an acid 
forming definite salts. It appears as common alum in the first of these 
states. If we take the well-known base magnesia, or lime, or oxide of 
zinc, and mix them with alumina in certain proportions, we get the 
minerals called spinels, the alumina acting as silica would under corre- 
sponding circumstances. 
There are various salts of alumina in nature. One of these is wavellite, 
or phosphate of alumina. Alumina may be precipitated easily by taking 
a soluble salt of alumina, dissolving it in water, and adding a littla 
