PHASES OF THE EARTH'S DEVELOPMENT. 297 



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° Fahren- 

 heit) 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. Geolo- 

 gists 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 con- 

 sidered, then, that temperatures higher than those of the electric fur- 

 nace 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 espe- 

 cially 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 



