METALS AND SOME OF THEIR COMPOUNDS 271 



gas absorbs hydrogen in consequence of the formation of solid 

 hydrides of the cerium metals. This solid alloy has the property 

 of sparking in a remarkable degree, and when rubbed against 

 a rough steel surface produces a shower of sparks which will 

 ignite a jet of coal-gas or the wick of a small lamp. 



Very large numbers of these pyrophoric metals have been sold, 

 especially in France, where matches are expensive, but these 

 contrivances can at present be regarded only as toys. 



COMPOUNDS OF METALS 



It is scarcely necessary to say that the metals by combining 

 with other elements produce a very large number of definite 

 compounds. Some of them yield several distinct oxides by 

 combination with different proportions of oxygen, and all metals 

 produce salts, which may be said to be their characteristic com- 

 pounds. Many of these, such as common salt, alum, green and 

 blue vitriol, have been known from the most ancient times, but 

 scores of new compounds of this kind are added to the list every 

 year, as the result of operations in the course of chemical research 

 or manufacture. Many metallic compounds are applied to useful 

 purposes as chemical agents, in medical practice, or as pigments 

 and otherwise. These, however, are the commonplaces of 

 practical chemistry, and information concerning them is stored 

 up in the larger textbooks and dictionaries of chemistry, and to 

 these sources of information the reader who desires it must be 

 referred. 



A few words may be added here as to the artificial production 

 of certain gems. This is the outcome of conveniences for the 

 production of high temperatures, such as the oxyhydrogen flame 

 and the electric furnace. It need scarcely be repeated that the 

 diamond is a form of carbon and that the diamonds produced 

 by Moissan's process (p. 83) so far are minute and are of no use 

 as gems. The ornamental stones, often very beautiful, which 

 are sold in imitation of diamonds are merely artificial silicates 

 containing lead and other heavy metals. Other stones such as 

 emerald, garnet, topaz, etc., are also imitated by glasses, but the 

 most interesting of artificial stones are the ruby and the sapphire 

 which are now produced commercially by the fusion of pure 

 alumina, of which both are essentially composed. The artificial 

 stones are not merely imitations, they are identical in hardness, 

 density, and crystalline form with the natural gems, Ruby 



