CHEMICAL SCIENCE. 201 



apparently infusible alumina into crystals similar to the ruby and the 

 other oriental stones. He proceeded by submitting to the action of 

 the blow-pipe a mixture of alum (sulphate of alumina and of potash) 

 and chromate of potash which he placed in a cavity of animal char- 

 coal. In this manner he obtained small portions of melted alumina, 

 having the color 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 colored like the precious ori- 

 ental stones have failed in a similar manner; and this has been 

 accounted for by the discovery only lately that the color of these 

 stones is not due to a metallic oxide, as had been always supposed, 

 but to the presence of some organic coloring matter. The applica- 

 tion 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 Ebehnen, who, ten years 

 after M. Gaudin'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 evaporating the liquid, it would be possible to obtain crystals 

 resembling 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 adhering so tightly that it was found 

 impossible to detach them entire. 



One other experiment is worthy of notice before proceeding to the 

 only one which had any practical result. It is that of M. de Senar- 

 mont, who obtained similar microscopic crystals by exposing hydrate 

 of alumina, or alumina combined with water, to a great heat, which 

 caused the water to evaporate, and left the crystals at the bottom of 

 the glass tubes in which the experiment was conducted. 



The perseverance of M. Gaudin, who appears never to have aban- 

 doned the idea of manufacturing precious stones, enabled him, in 1857, 

 to present to the Academy of Sciences several white sapphires, pro- 

 duced by a very simple process, and of sufficient size to be used as 

 jewels in watches. 



The following is the mode of procedure by which M. Gaudin suc- 

 ceeded in producing these crystals : 



In a crucible, lined with animal charcoal, are placed equal parts of 

 alum and sulphate of potash, previously calcined to expel the water. 

 With this mixture the crucible is hah'' filled ; it is then filled up to 

 the top with animal charcoal ; the lid is put on and cemented in its 

 place with clay, and it is then exposed in a furnace, and kept at a 

 white heat for a quarter.of an hour. The heat and the reducing power 

 of the charcoal cause the formation of sulphuret of potassium, which 



