Astronomers tell us that refractory elements like iron, silicon and carbon, perhaps disassociated into still simpler substances, are present as vapor in the atmosphere of the sun and that many others of our well-known elements, including hydrogen, are also present in this glowing atmosphere, while the heat of the sun’s surface and that of the hotter stars is vastly higher than that of the electric furnace. Geologists believe that the evidence at their disposal points to a similar period of great heat in the early history of the earth. It may be considered, then, that temperatures higher than those of the electric furnace prevailed in former times on the earth’s surface. Let us now return to the study of the results obtained with the electric furnace. The following reactions are especially important. If metals, or refractory non-metals, or metallic or non-metallic oxides, or complex silicates, are heated to the higher temperatures in contact with carbon, boron, silicon or compounds of these three elements with oxygen, the result generally is that very refractory carbides, borides or silicides of the metals or non-metals are formed. In other words, those complex substances which form the chief constituents of the outer crust of the earth at the present day are decomposed at high temperatures, and simple compounds of two elements--so-called binary compounds--are formed. Four classes of these binary substances seem to be especially stable at high heat--the carbides, borides, silicides and oxides; but the oxygen of the metallic oxides tends to pass off as an oxide of carbon, if carbon be present. At somewhat lower temperatures nitrogen is very active and the nitrides of many metals are readily formed. An excellent example is shown by heating a mixture of carbon and of an oxide of titanium (titanic acid). When heated by a feeble current the acid is simply reduced, forming a lower oxide of titanium; with a more powerful current the mass is completely changed into the nitride of titanium, the nitrogen coming from the air; with a very powerful current this is changed into pure carbide, as the nitride cannot exist at the higher temperature, and the nitrogen escapes, carbon taking its place. At still higher temperatures hydrogen acts on many metals, forming hydrides. The carbides and other compounds of some metals are not stable at high temperatures, being reduced by gaseous carbon to the free metals, which remain then in the gaseous form.