The effect of the heat on single substances is very interesting. Refractory metals, such as iron, manganese, uranium, platinum, melt rapidly and then become gaseous; the most refractory non-metallic elements, silicon, boron, carbon, are also changed into the gaseous form. Very refractory compounds are broken down into simpler ones. Magnesium pyrophosphate yields phosphorus, magnesium oxide and oxygen. Asbestos—a magnesium silicate—gives as chief product magnesium silicide; the other substances formed being silicon, silicon dioxide and a little magnesium oxide.
Such are the astounding changes wrought by simple heat upon those substances which we are accustomed to regard as infusible. It must be remembered that the range of temperature which chemists employ in ordinary laboratory work is not very great and that the conditions of work in the laboratory and of nature’s work on the earth’s surface at the present day favor the formation of two classes of compounds—the oxides and their hydrates. Although air is a mixture consisting mainly of four parts of nitrogen and one of oxygen, atmospheric nitrogen is generally inert at ordinary temperatures, and it is the oxygen of the air which is the more important factor in the growth of living things and in changes in lifeless matter. Water, a compound of oxygen and hydrogen, is present everywhere, either in the liquid form or as vapor in the air; even in the flame of the hottest fires there is water vapor in abundance, since water is one of the chief products of combustion of most forms of fuel. Is it a 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 be 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 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.
At that period of the earth’s history when the temperature was as high as that easily obtained in the electric furnace, we have the sanction of geologists for picturing the earth’s surface as an ocean of molten matter surrounded by a glowing atmosphere. This molten surface must have consisted of binary compounds such as those mentioned above, and probably contained some refractory elements, metals and non-metals, in the free state. The atmosphere contained free hydrogen, oxygen and nitrogen, gaseous binary compounds like the oxides of carbon, metals in the gaseous form and many non-metallic elements like sulphur and chlorine. In the atmospheric region furthest removed from the molten surface violent chemical reactions occurred between the heated elements, forming compounds which were again dissipated into their elements by the heat given off in the act of formation or radiated from the glowing surface below.
Under the enormous pressure of this atmosphere the liquid surface of the earth solidified at very high temperature. Whether the earth’s mass solidified from the centre outward or by forming a solid crust over a liquid interior, is a question to be decided by physicists and geologists. We will consider only the outer crust and the atmosphere. As the surface and the atmosphere above it gradually cooled, the formation of nitrides, and later of hydrides, sulphides and chlorides, occurred.
The conditions now attained may have been fairly stable as long as the temperature of the surface and lower regions of the atmosphere were high enough to prevent the union of the atmospheric oxygen and hydrogen, or to decompose the water forming in the outer regions of the atmosphere. As soon, however, as by further cooling, water came into contact with the earth’s surface, very violent reactions occurred, which were supplemented by other equally violent reactions when the cooling process permitted the formation of the ordinary mineral acids.
The reactions of water and of acids on many of the binary compounds are so important in determining the present composition of the earth’s crust that they must be considered in detail. The carbides, nitrides, chlorides, sulphides and hydrides of most elements, and some silicides, are decomposed by water, or else by dilute acids, forming the hydrogen compounds of carbon, nitrogen, chlorine, sulphur and silicon respectively, and the oxide or hydroxide of the other element. Thus calcium carbide and water give calcium hydroxide and acetylene, a hydro-carbon. Aluminum carbide yields alumina and methane (marsh gas), another hydro-carbon, the chief constituent of ‘natural gas.’ Other carbides yield crude petroleum. The nitrides yield ammonia, which is the hydrogen composed of nitrogen. The chlorides give hydrochloric acid, the sulphides sulphuretted hydrogen and the silicides the hydrogen silicide. The metallic hydrides yield free hydrogen.