186 Chemistry. [Lecture 30. 



where the salt forms a foot thick on the sides, 

 top, and bottom. The tub being unhooped, and 

 the staves removed, a hole is made in the lower 

 part of this crust, and the mother liquor (as it is 

 called) flows out into a proper receptacle, leaving 

 this artificial grotto lined and filled with large 

 crystals of the utmost beauty, which, however, 

 is forthwith demolished, and the fragments sent 

 off to the purchaser. 



Alum is of great use in the arts, particularly 

 in dyeing, where it acts as a vehicle for the 

 colour, or mordant, as the artists call it. It 

 serves as a ground to fix the colours in the' sub- 

 stance of the cloth, and is therefore used pre- 

 paratory to the stuff being put into the bath, or 

 vessel which contains the dye. It is generally 

 agreed that it is the earthy part of the alum 

 which has this effect, entirely independent of 

 the acid. 



Boles, red chalk, &c., are all mixtures of alu- 

 mina and other matters. Red chalk has its co- 

 lour from a vast quantity of iron which it con- 

 tains. 



III. The character by which SILICEOUS or 

 FLINTY earths and stones are distinguished is ex- 

 treme hardness, being impenetrable to the hardest 

 steel ; they make an impression on it, and strike 

 fire; they also make an impression on glass. 

 They are called crystalline, because found in 

 more regular crystals than other earths ; vitreous 

 or vitrescent, because used in making glass. 



