312 SILICON. 



Borates. Boracic acid is remarkable for the variety of pro- 

 portions in which it unites with the alkalies ; all these borates 

 have an alkaline reaction like the carbonates. The relative 

 proportions of oxygen and boron in boracic acid are known, 

 but the number of equivalents of these elements in this acid 

 is not so certain. Dumas inferred from the density of the 

 chloride that it is a terchloride, and boracic acid, which cor- 

 responds, will therefore consist of 3 eq. oxygen to 1 eq. boron, 

 and its formula be BO 3 . This makes borax the biborate of 

 soda. 



SECTION VI. 

 SILICON. 



Syn. SILICIUM. Eq. 277-31 or 22.22; Si O 3 ; density of 

 vapour (hypothetical) 1529; |~ I I* 



Silica or siliceous earth, the oxide of the present element, 

 is the most abundant of all the matters which compose the 

 crust of the globe. It constitutes sand, the varieties of sand- 

 stone and quartz rock, and enters into felspar, mica and a pro- 

 digious variety of minerals, which form the basis of .other 

 rocks. 



Preparation. Silica may be decomposed by heating it with 

 potassium, which deprives it of oxygen ; but a better process 

 for obtaining silicon, is to heat the double fluoride of silicon and 

 potassium, with 8 or 9-10ths of its weight of potassium, with the 

 same precautions as in the preparation of boron. The ma- 

 terials, however, in this case may be heated in a glass tube, as 

 well as in an iron cylinder. The double fluoride employed, is 

 prepared by neutralizing fluosilicic acid with potash. A different 

 process is suggested by Berzelius, which consists in heating 

 potassium in a tube of hard glass with a small bulb blown upon 

 it, which is rilled with the vapour of the fluoride of silicon, sup- 

 plied from the ebullition of that liquid contained in a small 

 retort connected with the glass tube. The potassium burns in 

 this vapour, and at the end, silicon is found, with fluoride of 

 potassium, in the place of the metal (Traite, t. 1, p. 137). But 

 the silicon from all these processes is always in combination 

 with a little potassium, and mixed with a little fluoride of 



