us 



THE GEOLOGIST. 



salt called chloride of aluminium, then we can, by means of a metal having 

 a powerful affinity for the chlorine, succeed in separating the aluminium 

 completely. 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 oxygen ; 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. 

 Yv r 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 Greenland. 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 3odium at a high tempe- 

 rature, we should produce aluminium. The thought occurred here some 

 time ago, and the very first specimen produced was 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, 533 per cent, of aluminium, and, consequently, 46"7 per cent, of 

 oxygen, and its chemical symbol is Al 2 0 3 . 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 little 



