138 THE ALKALIE8. 



their constitution in a strong heat. Notwithstancling tlic 

 varied appearance of the earth under our feet, and of the 

 mountainous parts of the world, whose diversified strata pre- 

 sent to our view substances of every texture and of every 

 shade, the whole is composed of only nine primitive earths ; 

 and as three of these occur but seldom, the variety which is 

 produced by the other six becomes the more remarkable. 

 One of the most valuable earths with which we are acquainted 

 is silex or pure flint. It is the most durable article- in the 

 state of gravel for the formation of roads ; it is a necessary 

 ingredient in earthenware, porcelain, and cements ; it is the 

 basis of glass, and of all vitreous substances. It is white, 

 inodorous and insipid in its pure state, and the various 

 colours, which it assumes in different substances, proceed 

 from the different ingredients with which it is mixed.. Alu- 

 mine obtained its name from its being the base of the salt 

 called alum. It is distributed over the earth in the form of 

 clay, and on account of its aptitude for moulding into dif- 

 ferent forms, and its property of hardening in the fire, is 

 employed for various useful purposes. In making earthen- 

 ware, a due proportion both of silex and alumine are neces- 

 sary ; for if alumine alone were used, the ware could not be 

 sufficiently burnt without slirinking too much, and even 

 cracking ; and a great excess of silex would lessen the te- 

 nacity and render the ware brittle. Lime is never found 

 pure in nature ; it is obtained by decomposing calcareous 

 matters by the action of fire, which deprives them of their 

 acid. In its pure state it is used in many of the arts. It is 

 employed by the farmers as a manure ; and by bleachers, 

 tanners, iron-masters and others, in their several manufacto- 

 ries, and in medicine. The use of lime in agriculture may 

 be attributed to its property of hastening the dissolution of 

 all animal and vegetable matters, and of imparting to the 

 soil a power of retaining a quantity of moisture necessary for 

 the nourishment and vigorous growth of the plants. Mag- 

 nesia, besides being the basis of several salts, is of great use 

 in medicine ; and is employed by the manufacturers of ena- 

 mels and porcelain. 



The alkalies are distinguished by an acrid and peculiar 

 taste ; they change the blue juices of vegetables to a green^ 

 and the yellow to a brown, and have the property of render- 

 ing oils nusciblq with water, They form various salts b^ 



