SECT, xxvir. EARTHS. 469 



parent of all : that which is of the best quality is 

 composed of 120 parts of white siliceous sand, 40 

 parts of pearl-ash, 35 of red oxide of lead, 13 of 

 nitrate of potass, and 25 of black oxide of manganese.* 

 It is known also by the name of crystal, and may be 

 cut and polished so as to serve for a variety of or- 

 namental purposes, as well as for the more important 

 and more useful purpose of forming optical instru- 

 ments, of which the discoveries of the telescope and 

 the microscope are the curious or sublime results. 



If a quantity of oil is mixed with half its weight 

 of a strong solution of soda or potass, a combination 

 takes place which is rendered more complete by 

 means of boiling. The new compound is soap. 

 The union of oil with potass forms soft soap, and 

 with soda hard soap ; substances of the greatest ef- 

 ficacy as detergents, and of the greatest utility in the 

 washing and bleaching of linen. 



The alkalies are used also in medicine, and are 

 found to be peculiarly efficacious in the reduction 

 of urinary calculi. 



SECTION XXVII. 



Earths. 



THE only earths which have hitherto been found 

 in plants are the following : lime, silica, magnesia, 

 alumina. 1st, Lime. Of these earths, lime is by 



* Accum's Chemistry, vol. ii. p. 194-. 



