150 
THE GEOLOGIST. 
is, that the fluorine goes away from the aluminium, and combines with 
the boron contained in the boraeic acid, and then escapes as fluoride 
of boron, and the oxygen of the boraeic acid unites with the aluminium, 
and forms the beautifully-crystallized substance corundum. Here, then, 
we can make sapphire in this way most beautifully. At present our 
artificers have not directed much attention to it, but there is no reason 
why, if we carried out this branch of investigation, many of our gems 
might not be produced iu this way. - 
These crystals are generally rhombohedral, terminated by the faces of 
the regular hexagonal prism, and according to Deville, they possess all 
the optical and crystallographical properties of natural corundum. Well, 
then, by just varying this process, we get the true oriental ruby — not the 
less valuable kind called spinel, but the true characteristic ruby — simply 
by adding a little fluoride of chromium. The process must be conducted 
in an alumina crucible, — the boraeic acid must be placed in a platinum 
cupel. Sapphire is produced in exactly the same manner, and the same 
colouring agent, namely, fluoride of chromium, is made use of. The rea- 
son of this is very obscure. Side b}^ side we may get some of the crystals 
coloured blue, and some coloured red. The difference is possibly due 
to a difference in the oxidation of the chromium, but that is a point not 
as yet clearly revealed by analysis. It is a very peculiar thing. The same 
thing occurs in certain experiments with glass. We know nothing by 
analysis of the colouring-matter of the sapphire. Whatever it is, it 
has escaped detection by analysis. Here, by means of the same colour- 
ing agents, and under tlie same conditions, or apparently under the same 
conditions, we produce sapphire and ruby. Sometimes we find this 
corundum passing into sapphire, beiiig blue in one part ; and jewellers, in 
setting such things, try with great skill to make it appear that the colour- 
less corundum is coloured, and they set it in such a way that the whole 
shall appear blue. By increasing the proportions of the chromium salt, 
Deville obtains a fine, rich, emerald green — an oriental emerald-green co- 
rundum. This is simply by increasing the quantity of the chromium. 
^ow, it is very peculiar that we can thus succeed in producing artificially 
one of the finest minerals, except the diamond. The diamond will come 
ultimately, no doubt. 
iS^ow, there is another process contrived by Debray, which appears to 
answer perfectly well. This consists of calcining phosphate of alumina 
with three or four times its weight of sulphate of soda or potash. He 
gets, then, tribasic alkaline phosphate and crystallized alumina. 
Alumina may be melted by oxygen and the flame of a spirit-lamp into 
glass-like beads, which are stated to be always more or less crystalline on 
cooling. Gaudin, especially, is the man who, many years ago, experi- 
mented upon this subject. The smallest globules show crystalline faces, 
and they are said to be so hard that their edges will cut glass. Sapphire, 
whether natural or artificial, is the hardest mineral known except the 
diamond. Alumina, when melted in the way spoken of, is said to be very 
liquid. By the addition of a very little chromate of potash during fusion, 
Gaudin obtained a more or less deeply-coloured red product, similar in 
colour to the natural ruby. It is reported to be more or less crystalline, 
and to possess extreme hardness. 
Now, how does corundum occur in nature? It is interesting to ascer- 
tain this point. It occurs frequently in limestone, forming the so-called 
stratified granite in Newton, in New Jersey, and in New York. It 
occurs in layers with marble, in the gneiss of the Isle of Naxos, and also 
