SILICON. 313 



silicon and potassium unreduced. Hence, on applying cold 

 water to the mass, hydrogen gas is disengaged, and potash 

 formed, and the silicon separates. The potash thus produced 

 can, with the aid of hot water, dissolve the silicon, which then 

 oxidates and becomes silica, so that cold water only must be 

 employed to wash the silicon, which may be thrown upon a 

 filter. After a time, the liquid which passes has an acid reac- 

 tion, which arises from its dissolving an acid double fluoride 

 of silicon and potassium, of sparing solubility, which has 

 escaped decomposition, and is mixed with the silicon. The 

 washing is continued so long as the water dissolves any- 

 thing. 



Properties. The silicon which is thus obtained is, in its 

 pure state, a dull brown powder, which soils the fingers, and 

 when heated in air or oxygen, inflames and burns, but is never 

 more than partially converted into silica. It may be ignited 

 strongly in a covered crucible without loss, and then shrinks 

 in dimensions, acquires a deep chocolate colour, and becomes so 

 dense as to sink in oil of vitriol. By this ignition the proper- 

 ties of silicon are altered to a degree which is very remarkable 

 in a simple substance. It was previously readily soluble in hy- 

 drofluoric acid, with evolution of hydrogen, and in caustic potash, 

 but it is now no longer acted upon by that or any other acid, 

 nor by alkalies. The ignited silicon also refuses to burn in air 

 or oxygen, even when intensely heated by the blow-pipe flame. 

 Charcoal, it will be remembered, is more dense and less com- 

 bustible after being strongly heated ; but that substance is not 

 altered by heat to the same extent as silicon. Mixed and heated 

 with dry carbonate of potash, silicon in any condition is 

 oxidated completely, its action upon the carbonic acid of 

 the salt being attended with ignition, and carbon liberated. 

 Silicon burns when heated in sulphur vapour, and forms a sul- 

 phuret, which water dissolves, but decomposes at the same 

 time, sulphuretted hydrogen and silica being produced, and 

 the last, despite its usual insolubility, retained in solution. 

 Silicon likewise burns in chlorine ; and the chloride of silicon 

 may be otherwise formed by transmitting chlorine over a mix- 

 ture of charcoal and silica ignited in a porcelain tube. The 

 silica is decomposed by neither charcoal nor chlorine singly, 

 but acting together upon the silica, these bodies produce car- 

 bonic oxide and chloride of silicon. This compound is a vola- 



