By the action of heat on what is termed ‘acid phosphate of lime,’ it is transformed into ‘crystallised pyrophosphate;’ and when heated to a still higher temperature, it passes into the vitreous or glassy state. It is supposed that in this condition it loses some of its phosphoric acid by volatilisation, and passes into the state of ‘tribasic phosphate.’ Such is the technical explanation of the changes which occur. The phosphate of lime glass is produced by taking this substance in a moist acid state, and heating it in an iron pot to a dark red heat. During this operation it is worked about with an iron rod, in order to prevent it swelling up and passing over the edge of the iron crucible. The dark red heat is continued until the whole mass has become glassy and transparent. At this moment it is run into another crucible, in which it is heated to a white heat that is kept up for about two hours, being stirred rapidly with a rod the whole time. At the end of this period the molten mass is allowed to remain perfectly quiet for about an hour, and is then run out of the crucible, either on to a metallic slab or into a metal mortar. It is necessary to avoid too rapid a cooling. The product may thus be run out into a sheet, like plate-glass. A small sheet of such a nature was obtained by M. Sidot in one of his experiments: it measured about three inches across, by a quarter of an inch thick, and was large enough to be cut into a considerable number of beautiful artificial sapphires. The ruby and sapphire have also been closely imitated in another way by Fremy and Feil, two French chemists; and the chief interest in this process is the fact that the artificial stones possess essentially the chemical composition of the real ones. To produce these, equal weights of alumina and red-lead are heated to a red-heat in an earthenware crucible. A vitreous substance is formed, which consists of silicate of lead, and crystals of white corundum. To convert this corundum into the artificial ruby, it is necessary to fuse it with about two per cent. of bichromate of potassium; whilst, to obtain the sapphire, a little oxide of cobalt, and a very small quantity of bichromate of potassium, must be employed. The stones so produced possess at least very nearly the hardness of the real stones, as they scratch both quartz and topaz.