160 THE WORLD'S LUMBER ROOM. 



and almost all fresh water, contains a small quantity of 

 salt. 



Common rock-salt is a compound of the gas chlorine 

 with the metal sodium, hence called chloride of sodium. 

 Various compounds of chlorine (chlorides) are found in 

 granite, mica, and many other minerals, as well as in 

 volcanic rocks; and compounds of sodium exist in such 

 enormous quantities that not a single minute speck of dust 

 is said to be free from them. 



Lava is frequently found covered with crystals of salt a 

 few days after it has been poured forth ; sometimes large 

 masses of salt are thrown out, and after some eruptions of 

 Mount Hecla enough has been found to load several horses. 



There is no mystery, therefore, as to whence the rivers 

 might be supplied with salt ; but as the quantity they convey 

 is usually too small to destroy their sweetness, it may at first 

 sight seem strange that it should be .enough to make the sea 

 so intensely salt. But if only a small quantity of salt is 

 being constantly added, while a much smaller quantity is 

 withdrawn, any body of water must become perceptibly 

 salt in time. We know that river water contains a minute 

 quantity of salt, though our taste is unable to detect it. 

 Sometimes, however, we can even taste it. 



During the great drought in La Plata, which lasted from 

 1827 to 1830, when the land became so loaded with dust 

 that old landmarks were obliterated, and much confusion 

 and even lawsuits were the consequence, evaporation went 

 on rapidly, and as, in addition to this, the rivers were fed 

 entirely by springs, many of the small ones became so salt 

 as to kill the animals which drank of them. 



