150 



THE GEOLOGIST. 



is, that the fluorine goes away from the aluminium, and combines with 

 the boron contained in the boracic acid, and then escapes as fluoride 

 of boron, and the oxygen of the boracic 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 in 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 boracic 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 by 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 the same conditions, or apparently under the same 

 conditions, we produce sapphire and ruby. Sometimes we find this 

 corundum passing into sapphire, being 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. 

 Now, 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. 



Now, 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 ver} r 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 



