CARBON, SILICON, BORON. 187 



furnishes a typical illustration of the possibilities of the electric furnace for 

 manufacturing purposes. Figs. 32 and 33, page 81, give a sectional and an 

 exterior view of the furnace used. The current enters and leaves the furnace 

 through cables terminating in carbon electrodes fastened in the wall. Between 

 them is placed a core of coke, surrounded by a mixture of carbon, sand, and 

 salt. The current heats the mass to about 3500 C. (6332 F.), when the 

 carbon combines with both elements of the sand, thus: 



Si0 2 + 3C = 2CO + SiC. 



Carborundum forms beautiful, dark-green, iridescent crystals of extreme hard- 

 ness, in the latter quality being exceeded only by the diamond. It is extensively 

 used as a polishing agent, gradually replacing emery, to which it is far superior. 



Boron, B'" = 10.9, is found in but few localities, either as boric 

 (boracic) acid or sodium borate (borax). Formerly the total supply 

 of boron was derived from Italy : large quantities of borax are now 

 obtained from Nevada and California. 



Boron exists as a greenish -brown amorphous powder and also in the form 

 of hard and often highly lustrous crystals. Boron combines with many of the 

 non-metals, forming such compounds as boron trichloride, BC1 3 , trifluoride, 

 BF 3 , and hydride, BH 3 . It is one of the few elements which at a high tempera- 

 ture combine directly with nitrogen, forming nitrogen boride, BN. 



Boric acid, Acidum boricum, H 3 BO 3 , B(OH) 3 = 61.54 (Boracic 

 acid), is a white, crystalline substance, which is sparingly soluble in 

 cold water or alcohol, but more soluble in glycerin ; it has but weak 

 acid properties. When heated to 100 C. (212 F.) it loses water, 

 and is converted into mdaboric acid, HBO 2 , which when heated to 

 160 C. is converted into tetraboric acid, H 2 B 4 O 7 , from which borax, 

 Na 2 B 4 O 7 -f 10H 2 O, is derived. At a white heat boric acid loses all 

 water, and is converted into boron trioxide, B 2 O 3 . From a boiling 

 solution boric acid readily volatilizes with the steam. 



Boric acid is obtained by adding hydrochloric acid to a hot satur- 

 ated solution of borax, when boric acid separates on cooling. The 

 chemical change is this : 



7 + 2HC1 + 5H 2 O = 4H 3 B0 3 + 2NaCl. 



It is rather odd that while the usual form of boric acid in the uncombined 

 state is the orthoboric acid, H 3 BO 3 , salts of this form are hardly known. Salts 

 of metaboric acid occur, but the best-known salts are derived from tetraboric 

 acid, and the best representative is the sodium salt, borax. On the other hand, 

 when the acid is liberated from the salts, it assumes the ortho form. From the 

 formula of tetraboric acid, it appears that four molecules of the ortho acid com- 

 bine with elimination of water to form a more complex molecule : 



4H 3 B0 3 H 2 B 4 7 + 5H 2 0. 



There are other cases of this kind, as will be seen later. 



