SPECIAL PROBLEMS 117 



Corrosion of Casings. Many waters attack casings with great 

 rapidity and destroy them in the course of a few years, some- 

 times even within a few months, although other casings may last 

 for 10 to 20 years. In general it has been found that the waters 

 attacking the casings most rapidly are those which are highest in 

 carbon dioxide. Such waters attack zinc as well as iron and the 

 use of galvanized pipes affords little relief. Block-tin has been 

 occasionally substituted, but sometimes even this has been eaten 

 through, although it is ordinarily little affected by water. In 

 places, relief has been obtained only when wooden pipe has been 

 substituted, although the life of such piping is short and its 

 use is otherwise objectionable. Of the iron pipes, the common 

 " black iron " type seems to give the least trouble. 



