6o VAIL : 



been made of silicon, which would serve the purpose even 

 better, but the present cost of reducing silica is too great to 

 admit of their use. Carbon and silicon both combine with 

 many metals directly, to form carbides and silicides respect- 

 ively. These have been much studied in their relation to the 

 properties of iron and steel, in which they almost invariably 

 occur. Notable among commercial carbides is that of calcium, 

 the source of acetylene for lighting purposes ; while carbo- 

 rundum, the great abrasive, is a silicide. 



As yet no form of silicon comparable to the diamond has 

 been discovered. 



In October, 1907, Mr. E. G. Acheson delivered a lecture 

 before the Franklin Institute on the subject of " Deflocculated 

 Graphite," in which he explained how graphite may be sus- 

 pended in water in a state of division much finer than mecha- 

 nical means could produce. This is accomplished b}- means 

 of a small amount of tannic acid digested with the finely 

 divided graphite, and the product will pass through the finest 

 filter. He stated that the effect may be obtained with many 

 non-metallic, amorphous bodies. In this connection it is 

 interesting to note that graphitoidal silicon shows almost iden- 

 tical action with graphite under this treatment. It is sus- 

 pended in the water so as to pass the finest filter, and is 

 thrown out from the state of suspension on acidifN-ing with 

 hydrochloric acid. When the water containing the suspended 

 silicon is evaporated we get, not the graphitoidal form with 

 which we started, but amorphous silicon, which can be burned 

 in the air. Graphitoidal silicon cannot be burned in air or 

 oxygen. 



Chemically, carbon is the most interesting of all the ele- 

 ments on account of the vast number of compounds which it 

 is capable of forming. We are told that this is explained by 

 the ability of the carbon atoms to combine with each other to 

 form chains. It is by studying the structure of the molecule 

 that we can classify and investigate organic compounds. 

 Beside analysis, which reveals only the percentage composi- 



