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.