of two elements, the gas oxygen and the hard, light, non-metal silicon. It is a substance that is almost omnipresent in the rocks of the world. In organisms, on the other hand, it is the carbon which is the chief element, and about which the hydrogen and oxygen and nitrogen group themselves. Silicon seems, then, to play much the same rolé in the mineral world that carbon does in the organic. In many respects the two elements themselves are similar. But the point of interest lies in their compounds. Fine, crystallized carbon, the diamond, is not readily altered. Nor are its simpler compounds with the elementary gases, such as carbon dioxide, marsh-gas, and cyanogen. But when the compounds become more complex, when carbon unites with all three of these elements, and the molecule contains many so-called atoms, it is correspondingly unstable. The highest development of this complicated organic structure is found in the human brain, and in the rapid changes which go on in these tissues we have, if not the cause of thought, at least its accompaniment. The quality and quantity of thought apparently depend upon the differentiation of these carbon compounds, and the consequent ease and rapidity with which they can decompose and recompose.
Now, we have in the mineral world at least a partial parallel to this general behavior, and one that is well illustrated in the members of the mica family. Silicon itself is never found alone, and the proximate reason for this is readily understood. It is a fundamental law of chemistry that, when two reactions are possible, that one will take place which will liberate the greater amount of heat. Apply this to silicon. When it unites with oxygen, the heat of combination is very great, greater than that produced by the combination of oxygen with carbon, and consequently this reaction would take place in preference to many others, even in preference to the oxidation of carbon. The point is admirably illustrated by the chemical reactions taking place in the Bessemersteel process. The pig iron which is run into the converter consists in the main of metallic iron combined with carbon and silicon. When the blast of air bubbles through the molten metal, it is the silicon which first oxidizes. The flame escaping from the mouth of the converter is small and intensely hot. The spectroscope shows a predominance of the silicon lines. Then the carbon flame appears, less hot and more voluminous—the second choice of the oxygen. Finally, the iron itself begins to burn and the blast is discontinued. Bearing these facts in mind, we would never expect to find free silicon, and we are never disappointed. When the element combines with oxygen, in silica or quartz, we have a simple and extremely stable compound, as with the corresponding carbon compound. At a high heat and in the presence of metallic bases, the silica will readily enter into new combina-
