1909] oil Advances in Knoivledge of Silicon. 643 



earth cooled down, and the product is the crust on which we live.* It 

 is probable that the proportion of oxygen diminishes away from the 

 surface until it disappears almost wholly. What of the deeper 

 depths ? Are the comparatively light elements arranged more or less 

 in the order of density ? Are we to suppose that silicon and some 

 carbon, aluminium, calcium, the elemeuts chiefly comprising the 

 crust, are those nearer the surface, and iron, copper, and the heavier 

 metals nearer the centre ? 



Until recently we knew little more than that the earth is some 

 8000 miles in diameter ; that its mean density is 5"6-5'7, and that 

 its relatively thin outer skin, or crust, has approximately the com- 

 position assigned to it in the diagram. By a very skilful use of 

 earthquake observations, the eminent geologist, Mr. Richard D. Oldham 

 has, however, lately! given us something like a glimpse within the 

 ball, and concludes from his observations that about five-sixths of the 

 earth's radius includes fairly homogeneous material and that the re- 

 maining sixth at the centre consists of substances of much higher 

 density. Assuming this to be even roughly true, we conclude that 

 silicon forms probal)ly as great a ]»roportion of this large mass of 

 the earth — whether in the free state or in tbe forms of silicides— as it 

 does of the crust. 



Having thus magnified the office of the important element of 

 which I wish to speak to you, I shall pass to my next point which is 

 how the element can ])e separated from quartz, or other forms of the 

 oxide, for it is never met Avith unless combined with oxygen in any of 

 the rocks known to us. 



I have already mentioned that quartz is a dioxide of the element 

 — in fact it is the only known oxide — hence if we remove this oxygen 

 we should obtain free silicon. This is not a very difficult matter as it 

 is only necessary to heat a mixture of finely powdered quartz with 

 just the right proportion of metallic magnesium. The metal com- 

 bines with the oxygen of the quartz, and forms therewith an oxide of 

 magnesium, while silicon remains. If the material be heated in a 

 glass vessel the moment of actual reduction is marked by a bright 

 glow which proceeds throughout the mass. When the product is 

 thrown into diluted acid the magnesium oxide is dissolved and nearly 

 pure silicon is obtained as a soft dark l)rown powder which is not 

 soluble in the acid. This is not crystalline, but if it be heated in an 

 electric furnace it fuses and on cooling forms the dark crystalline sub- 

 stance on the table, which, as you see, reseml^les pretty closely the 

 graphitic form of carbon, though its density is rather greater. (2'6, 

 graphite being 2:-).) 



* An interesting calculation has been made by Mr. Gerald Stoney, from 

 which it appears that a stratum only 9 feet in depth of the surface of the earth 

 contains as much oxygen as the whole atmosphere. See Phil. Mag., 189^*, p. 566. 



t R. D. Oldham, F.G.S., " Constitution of the Interior of the Earth." 

 Quarterly Journal of the Geological Society, vol. Ixii. (1906) pp. 456-475. 



