Salts. 147 



clear from the water, for it now forms a curd on 

 the surface of the fluid, which may be drawn off' 

 from under it by a pipe at the bottom of the 

 vessel. 



More salts are referred to the class of acids 

 than to that of alkalies ; but at present we shall 

 turn our attention to the three principal mineral 

 acids, called mineral because originally extracted 

 from mineral substances ; these are the sulphuric, 

 nitric* and muriatic. The form in which we 

 generally obtain them is in that of a watery fluid . 

 In this state they must be considered as volatile 

 with respect to other salts, as the most fixed of 

 them does not require above 600 degrees to con- 

 vert it into vapour ; the others become volatile 

 with a much less degree of heat. They have a 

 greater attraction for water than potass, or any 

 other substance. Their union with it is attended 

 with great commotion and heat ; though when 

 mixed with ice or snow they produce cold. 

 The nitric acid produces the most intense cold. 

 Some have imagined that the sulphuric acid 

 produces heat with ice; but they have deceived 

 themselves in making the experiment, by mix- 

 ing too much acid with the ice. No more acid 

 should be added than what is necessary to li- 

 quefy the ice ; for, after the ice is melted, the 

 acids produce heat in proportion to the quan- 

 tity of acid mixing with water. When diluted 

 with water, the mineral acids can only be dis- 

 tilled to a certain degree of concentration. It is 



H 2 



