OXY-SALTS AND HALOID SALTS. 51 



oration from the water of salt springs. The springs in 

 Onondago county, New York, are the most productive, 

 one seventh part of the water being salt. In hot climates 

 much salt is obtained from sea water by evaporation. 

 The manner in which this is done is described in 358, 

 Part II. 



99. Borate of Soda. Nearly naif (47 per cent.) of this 

 salt, commonly called borax, is water. It abounds in a 

 lake in Thibet, where it is literally dtfg out from the bor- 

 ders and shallow places. As the borax is deposited from 

 the water, the holes made in gathering it are soon filled 

 up. This lake is at so high an elevation that it is frozen 

 over the greater part of the year. The acid which en- 

 ters into the composition of borax, boracic acid, is some- 

 times found in the vicinity of volcanoes, exhaling from 

 springs in the earth. It is present in the hot vapors of 

 the lagoons in Tuscany, from which it is obtained by let- 

 ting the vapor pass into cold water to condense it, and 

 then allowing the solution to evaporate. This leaves the 

 boracic acid in large crystalline flakes. 



100. Carbonate of Lime. This mineral, in the forms of 

 common limestone, chalk, and marble, constitutes about 

 one seventh part of the crust of the globe, and therefore 

 will be brought quite largely to your notice in the geo- 

 logical part of this book. There are mountains made of 

 it, and there are extensive deposits of it in layers of rock. 

 It appears also in smaller quantities in many other forms, 

 some of them exceedingly beautiful. It is readily distin- 

 guished from other minerals by dropping some acid, as 

 the sulphuric, upon it, this occasioning an effervescence 

 by uniting with the lime and setting free the carbonic 

 acid, as already stated in 60. Exposed to a red heat 

 the carbonic acid is driven off, and we have the lime left 

 alone. This is what is done in the lime-kiln, and it is in 

 this way that the lime used for making mortar and other 

 purposes is obtained. Hydraulic lime, which has this 

 name because it will "set" under water, is made from 



