SILICON; BORON 363 



suffers very little change in volume with change in temperature, 

 and so unequal cooling causes almost no strain. 



Pyrex glass, a borosilicate, has also come into extensive use 

 lately for laboratory ware and for cooking vessels. Its low co- 

 efficient of expansion renders it much less liable to crack under 

 sudden temperature changes. Like pure silica, it is very re- 

 sistant to chemicals. It possesses the further advantage of 

 withstanding much greater mechanical shocks. 



BORON B 



The element boron resembles silicon and graphite in appearance. 

 It has no applications. 



Boric Acid H 3 BO 3 . This acid is contained in the steam which 

 issues from the ground in certain parts of Tuscany. It is caught 

 in water, placed in basins built over the " soffioni," and separated 

 by evaporation. Much of it is also made from borax. 



Boric acid crystallizes in white, slippery scales. It dissolves 

 somewhat in water (4 : 100 at 18), and the saturated solution, 

 mixed with an equal volume of water, is used as an eye-wash. 

 Boric acid is an exceedingly weak acid; its solution scarcely 

 affects litmus. It is a mild antiseptic, and preserves foods by 

 preventing the development of moulds and bacteria. It is often 

 added to talcum powder to prevent infection of irritated skin. 



When heated, it loses water and gives tetraboric acid: 



Skeleton: H 3 B0 3 - H 2 B 4 7 + H 2 O 



Balanced: 4H 3 B0 3 -* H 2 B 4 7 + 5H 2 O 



and eventually boric anhydride B 2 3 . 



Borax. This salt is the decahydrate of sodium tetraborate 

 Na 2 B 4 7 ,10H 2 0. 



It is made by adding calcium borate, found in California, 

 to sodium carbonate solution. The precipitate of calcium car- 



