246 TOWERS AND TANKS FOR WATER-WORKS. 



and acts upon the new surfaces of the metallic iron, and 

 owing to the porous and hygroscopic character of the rust 

 crust, only small quantities of oxygen and moisture are neces- 

 sary to indefinitely continue the process, the hydrated oxide 

 giving no protection to the underlying metal. The capacity 

 of rust for absorbing and condensing moisture and oxygen is 

 enormous, and it has been proved that iron-rust will absorb 

 as much as 27 gallons of oxygen-gas in making one pound of 

 rust. 



It seems beside the strictly chemical action, there is a 

 galvanic effect which augments the work of corrosion and 

 destruction when once begun ; for it lias been shown that the 

 oxides of any metal are electro-negative to the metal itself, 

 and that in ferric oxide a voltaic action is set up in its fibres 

 and surfaces in contact by thermo-electric currents due to 

 changes of temperature of the body ; further, that the contact 

 of such products as iron and steel is sufficient to set up such 

 action, the result being a pitting and corrosion of the material, 

 now technically known as electrolysis ; and it has been asserted 

 that the difference in the molecular arrangement of the same 

 materials due either to manufacturing methods which result 

 in lack of homogeneity, or from the unequal application of 

 force as stress that changes the arrangement of the fibres is 

 sufficient to produce voltaic destructive action. 



Mill-scale. In rolling iron or steel, the scale sometimes 

 left upon the surface of the metal, and known as " mill-scale," 

 has been analyzed as sesqui-oxide of iron, Fe* O', the same chem- 

 ical composition as ordinary iron-rust, and it seems further to- 

 possess to the same marked degree the capacity for absorption 

 and condensing moisture and oxygen, producing corrosion 

 and decay, and setting up galvanic action, the effect appearing 

 in rust-cones pitting and eating the metal. 



It is asserted that where mill-scale is left upon plates of 



