298 POPULAR SCIENCE MONTHLY. 



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 com- 

 pounds 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 



