316 SULPHUR. 



nate, and afterwards dissolving in dilute acid. The solution, 

 on concentration, gives a transparent jelly, which is highly 

 tenacious, and cracks on drying, forming a mass like gum. 

 When completely dried in the air, the mass is no longer so- 

 luble in water or acids. It contains a small quantity of water^ 

 which however, according to Berzelius is hygroscopic ; silica 

 affording him no definite hydrates, like those of other acids. 

 But I should still be disposed to look to the state of hydration, 

 however feebly the water may be retained, for an explanation 

 of the differences between the soluble and insoluble varieties 

 of silica. Hydrofluoric acid is the only acid which dissolves 

 silica in both conditions. 



Silicates. Although silica has no acid reaction, it is cer- 

 tainly an acid, and is indeed capable of displacing the most 

 powerful of the volatile acids at a high temperature. It is 

 capable of uniting with metallic oxides, by way of fusion, in a 

 great variety of proportions. Its compounds with excess of 

 alkali, are caustic and soluble, but those with an excess of silica 

 are insoluble, and form the varieties of glass, which will be no- 

 ticed under the silicate of soda. With alumina it forms the 

 less fusible compounds of porcelain and stoneware which will 

 be noticed under that earth. A large number of mineral spe- 

 cies are also earthy silicates. It seems probable that silicic, 

 like phosphoric acid, forms several classes of salts, of which 

 those containing the largest number of atoms of base are the 

 most soluble, and afford, when decomposed the soluble silica. 

 At the same time some difference may exist between the silicic 

 acid itself, as it exists in these different classes of salts, such 

 as there is between ignited and unignited silicon. 



SECTION VII. 

 SULPHUR. 



Eg. 201.17 or 16.12; S; density 6648; combining measure 

 l-3rd. volume. 



This element is exhaled in large quantity from volcanoes, 

 either in a pure state or in combination with hydrogen, and by 

 condensing in fissures forms sulphur veins, from which the 

 greater part of the sulphur of commerce is derived. It exists 



