a tetravalent nonmetallic element; next to oxygen it is the most abundant element in the earth's crust; occurs in clay and feldspar and granite and quartz and sand; used as a semiconductor in transistors, Chemical symbol, Si; atomic weight, 28. 19. A non-metallic element which is obtained in three allotropic forms?namely, amorphous, as a dull-brown powder soluble in alkali, which burns when ignited; graphitic, in crystalline leaves having a strong metallic luster and lead-gray color, insoluble in alkali and non-combustible; and crystalline, in octahedral needles having a red luster, and hardness a little less than that of the diamond, Elementary silicon can now he prepared in large qnantity by electrolysis, and in a fused condition. Like its oxid, in cooling from a state of fusion it passes through a plastic stage in which it can be molded into special forms or drawn into threads