WATER 



FIG. 32. From the mingling of two liquids a 

 solid is sometimes formed. 



66. Constructive Action of Water. Water does not always 

 act as a destructive agent ; what it breaks down in one place 

 it builds up in another. It does this by means of precipita- 

 tion. Water dissolves salt, and also dissolves lead nitrate, but 

 if a salt solution is mixed with a lead nitrate solution, a solid 

 white substance is formed in the water (Fig. 32). This for- 

 mation of a solid substance 

 from the mingling of two 

 liquids is called precipita- 

 tion ; such a process occurs 

 daily in the rocks beneath 

 the surface of the earth. 

 (See Laboratory Manual.) 

 Suppose water from dif- 

 ferent sources enters a 

 crack in a rock, bringing 

 different substances in so- 

 lution ; then the mingling of the waters may cause precipita- 

 tion, and the solid thus formed will be deposited in the crack 

 and fill it up. Hence, while ground water tends to make rock 

 porous and weak by dissolving out of it large quantities of 

 mineral matter, it also tends under other conditions., to rriake 

 it more compact because it deposits in cracks, crevices, and 

 pores the mineral matter- precipitated from solution. 



These two forces are constantly at work ; in some placed the 

 destructive action is mere prominent, in other places the con- 

 structive action ; but always the result is to change the charac- 

 ter of the original substance. When the mineral matter pre- 

 cipitated from the solutions is deposited in cracks, veins are 

 formed (Fig. 33), which may consist of the ore of different 

 metals, such as gold, silver, copper, lead, etc. Man is almost 

 entirely dependent upon these veins for the supply of metal 

 needed in the various industries, because in the original con- 



