ARTIFICIAL PRECIOUS STONES. 
333 
others proved, however, that it was pure alumina, the silica 
found by Chenevix being abraded from the substance in which 
the stones were imbedded. All the varieties of corundum 
crystallize in six-sided prisms, and have the curious property 
of double refraction ; i. e., causing everything that is looked at 
through them to appear double. Alumina, the oxide of the 
metal aluminium now coming into such frequent use in the 
manufacture of articles of jewellery, &c., was, until the inven- 
tion of the oxyhydrogen blowpipe, supposed to be, like carbon, 
infusible by any degree of heat. In 1837, however, M. Gaudin, 
who had given much attention to the effects produced by this 
then newly-invented means of generating heat on various 
metallic oxides formerly thought unsusceptible of fusion, 
attempted with some success to convert, by its aid, the ap- 
parently infusible alumina into crystals similar to the ruby and 
the other oriental stones. He proceeded by submitting to the 
action of the blowpipe a mixture of alum (sulphate of alumina 
and of potash) and chromate of potash, which he placed in a 
cavity of animal charcoal. In this manner he obtained small 
portions of melted alumina, having the colour and hardness of 
the ruby, but which could be easily distinguished from it by 
their imperfect transparence, and by their not possessing the 
property of double refraction. All subsequent attempts to 
obtain crystals of alumina, coloured like the precious oriental 
stones, have failed in a similar manner ; and this has been 
accounted for by the discovery only lately that the colour of 
these stones is not due to a metallic oxide, as had been always 
supposed, but to the presence of some organic colouring 
matter. The application of this discovery may bring us nearer 
than we have ever yet been to the invention of a mode of 
producing artificially these rare gems. 
The next step in this direction was made by the manager of 
a manufactory of Sevres porcelain, named Ebelmen, who, ten 
years after M. Gaudhr’s experiments, found out a way of 
obtaining crystals of corundum, but of such minute proportions 
as to be of no practical use. He first discovered that boracic 
acid, which had been hitherto supposed to be absolutely fixed, 
could be evaporated by the intense heat of the porcelain ovens ; 
upon this it occurred to him that by dissolving alumina in 
boracic acid, which could be done by heat, and then evapo- 
rating the liquid, it would be possible to obtain crystals re- 
sembling the oriental stones ; and it was found, in fact, that 
by exposing a platinum capsule containing such a mixture to 
the heat of the porcelain oven for a considerable time, the 
boracic acid was evaporated, and a number of little shining 
crystals of alumina having the properties and appearance of 
small precious stones were left adhering to the capsule, but 
