Mixler — Combustion of Silicon and Silicon Carbide. 139 



It was observed that the products of some of the combustions 

 of silicon gave off a gas when digested with cold dilute hydro- 

 fluoric acid. This gas was presumably hydrogen or a hydro- 

 carbon resulting from the interaction of a soluble modification 

 of silicon or a silicon compound and the acid. This is the 

 most probable explanation of the wide variation in the results. 

 The substance noted as silicon carbide in the foregoing table 

 was the residue insoluble in nitrohydrofluoric acid. It was 

 repeatedly found to yield carbon dioxide when fused with lead 

 chromate and in one portion 6 8 *7 per cent of silicon was ob- 

 tained : calculated, 70*3 per cent for SiC. The carbide from 

 the combustions was in the form of a light grey amorphous 

 powder. 



The heat of oxidation of silicon ma} 7 be calculated as 

 follows : 



SiC, 20„ = 283750*= 



Si, C = 1963 c 



285713 

 C, 2 = 94728° 



Si, 2 = 1909S5 C 



Berthelot's* result is 180593 c for the formation of gelatinous 

 silicic acid from 28*4 grams of silicon. 



Lithium orthosilicate is known and it is probable that sodium 

 orthosilicate results when silicon burns in an excess of sodium 

 peroxide. On this assumption the thermal effect of the combi- 

 nation of one molecule of silicon dioxide with two molecules of 

 sodium oxide may be calculated thus : 



2Na„0 2 , Si = 237000 



2Na"0 -0„ = 38000 c 



275000 

 Si, 0„ = -191000 



2Na 2 0, Si0 2 == 84000° 



Experiments were made to determine the heat of combina- 

 tion as follows: Silicon dioxide in the form of a bulky impal- 

 pable powder, obtained by igniting silicic acid resulting from the 

 decomposition of silicon fluoride in water, was used. A mixture 

 of sodium peroxide, carbon sufficient to form an excess of 

 sodium oxide, and silicon dioxide was ignited in the bomb. The 

 temperature of the reaction was sufficient to melt silver. Less 

 than 50 cc of oxygen were evolved. No free silica was found 



*Loe. cit. 



