WHAT TO USE FOR WATER-PIPES. 137 



suit of galvanic action. The oxide of lead is a sol- 

 uble compound, and quite poisonous. If the results 

 of the contact of water with lead were to stop here, 

 not a family could use leaden pipes with impunity. 

 The oxide would continue to form as fast as it was 

 washed away and dissolved by the current, and 

 shortly the whole structure would be destroyed. 

 But most waters contain, or hold in .solution, another 

 element, carbonic acid, which readily combines with 

 the oxide, and forms a new salt. This is the car- 

 bonate of lead, and fortunately is insoluble. The 

 first action, then, of most waters upon lead is to 

 form upon the surface a coating of the white oxide 

 of lead ; the second action is to change this danger- 

 ous soluble oxide into a hard insoluble carbonate, 

 and this, adhering to the whole interior surface of 

 leaden pipes, prevents further contact of the water 

 with the metal, and all decomposition ceases. This 

 is a plain statement of the way in which lead is usu- 

 ally acted upon by water ; and if there were no dis- 

 turbing agencies to come in and interfere with these 

 results, we should hardly require safer or better 

 water-pipes than those constructed of lead. 



It sometimes happens that well and spring waters 

 contain other agents which interfere with the chem- 

 ical changes we have described. Many wells in the 

 Northern States are fed by springs which percolate 

 through or over beds of rocks charged with iron 



