Alumina itself is so refractory that it cannot be melted save by the oxyhydrogen blowpipe or the electric arc, and except in the molten state it is not susceptible of decomposition by any chemical reagent.
It is infusible before the gas blowpipe, but in the oxyhydrogen flame fuses to a clear colourless glass, which has a hardness of 5 and specific gravity 2.2.
This powder, provided that it has not been too' strongly ignited, is soluble in strong acids; by ignition it becomes denser and nearly as hard as corundum; it fuses in the oxyhydrogen flame or electric arc, and on cooling it assumes a crystalline form closely resembling the mineral species.
Fizeau he carried on a series of investigations on the intensity of the light of the sun, as compared with that of carbon in the electric arc, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of heat rays, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light.
Paschen proved that the emission spectra of water vapour as observed in an oxyhydrogen flame, and of carbon dioxide as observed in a hydrocarbon flame may be obtained by heating aqueous vapour and carbon dioxide respectively to a few hundred degrees above the freezing point.
Many forms of oxyhydrogen lamps have been invented, but the explosive nature of the gaseous mixture rendered them all more or less dangerous.
The powdery metal burns readily in air; the crystalline metal requires to be heated in an oxyhydrogen flame before it catches fire.
The metal as obtained in this process is lustrous and takes a polish, does not melt in the oxyhydrogen flame, but liquefies in the electric arc, and is not affected by air at ordinary temperatures.
In the oxyhydrogen flame silver boils, forming a blue vapour, while platinum volatilizes slowly, and osmium, though infusible, very readily.
It acquired considerable application in platinum works, this metal being only fusible in the oxyhydrogen flame and the electric furnace; and also for the production of limelight, as in optical (magic) lanterns.