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.