wonder that under such conditions we find the earth's crust to contain the elements chiefly compounded with oxygen? Was this always so? Are we justified in supposing that conditions may have prevailed—nay, must have prevailed—in former times on the earth's surface, which gave to other elements as important or more important functions than to oxygen? The answer to these questions must he sought in the results of the chemistry of high temperatures.
First let us consider the conditions of existence of the omnipresent water. Water begins to break down into its components, hydrogen and oxygen, at 934° centigrade; at 2,500° centigrade (4,500° Fahrenheit) the decomposition is complete. In other words, water vapor cannot exist at temperatures above 2,500°, but the hydrogen and oxygen exist in the free state.
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
