SILICATES OF SODA. 4/9 



quantity of carbonic acid which escapes indicates the formation 

 of a borate, 3NaO -f 2BO 3 , but which has not been farther ex- 

 amined. 



A salt is said to exist, formed of NaO+4BO 3 , but to crys- 

 tallize with difficulty, formed on combining borax with a quantity 

 of boracic acid equal to what it already contains. M. Laurent 

 has also shewn that a sexborate of soda exists in solution, but is 

 not cry stall! zable*. The borates of potash have also been ex- 

 amined by Laurent. The sexborate crystallizes well ; its for- 

 mula is KO, 6BO 3 + 10HO. A triborate is represented by 

 KO, 3BO 3 + 8HO; the biborate corresponds in composition 

 with octohedral borax, but has, notwithstanding, a different and 

 incompatible form. 



Silicates of soda. When the earth silica (page 314) is 

 thrown into carbonate of potash or soda, in a state of fusion 

 by heat, a fusible silicate is formed, in which, judging from 

 the quantity of carbonic acid expelled, 3 eq. of alkali are com- 

 bined with 2 of silica, or the oxygen in the alkali is to that in 

 the silica as 1 to 2. This silicate dissolves in the clear and 

 liquid carbonate. When on the other hand a greater proportion 

 of silica is fused with the carbonate, the whole carbonic acid of 

 the latter is expelled, and the excess of silica then dissolves in 

 the silicate. The silica and silicate of such mixtures do not 

 separate by crystallization, but uniformly solidify together, on 

 cooling, as a homogeneous glass, whatever their proportions 

 may be. It is thus impossible to obtain alkaline silicates, which 

 are certainly definite combinations. A mixture of silica with 

 potash or soda, in which the oxygen of the former is to that of 

 the latter as 18 to 1, is said still to be fusible by the heat of a 

 forge ; but when the proportion is as 30 to 1, the mixture 

 merely agglutinates or frits. These combinations, even with a 

 large quantity of silica, continue to be soluble in water. 



A compound, known as soluble glass, is obtained by fusing 

 together 8 parts of carbonate of soda (or 10 of carbonate of 

 potash) with 15 of fine sand and 1 of charcoal. The object of 

 the charcoal is to facilitate the combination of the silica with 

 the alkali, by destroying the carbonic acid, which it converts 

 into carbonic oxide. This glass, when reduced to powder is not 

 attacked by cold water, but is dissolved by 4 or 5 parts of 



* An de Ch. et do Ph. t. 67, p. 218, 

 I I 



