PAINTING. 245 



duce the phenomenon of rusting until water is added to com- 

 plete the compound. Fresh water alone, when free from 

 acids or organic impurities, has been found to have but little 

 effect upon submerged plates of bright iron or steel, but where 

 the plate is entirely or intermittently immersed in salt water, 

 the salt water, taking the iron oxides into solution, removes 

 the oxides and exposes fresh metallic surfaces to attack, also 

 setting up a voltaic action upon ferric bodies. 



Structural work is generally exposed only to atmospheric 

 action, the atmosphere being sometimes charged with salt- 

 sea vapors, and always with some moisture, in addition to 

 the three universal components nitrogen, oxygen, and car- 

 bonic acid gas in the presence of which the destruction of 

 ferric members is sure ; the intensity and extent of this action 

 being directly dependent upon the quantities of each element 

 entering into the chemical action. 



Chemical and Galvanic Action. The chemical reaction 

 in such cases is the setting free of the hydrogen of the water, 

 its oxygen, uniting with the carbonic acid and metal, forming 

 ferrous carbonate, which again combining with the oxygen of 

 the water or atmosphere, is decomposed into ferric oxide and 

 carbonic acid gas, the latter passing off, leaving the sesqui-oxide 

 of iron to absorb and condense water, becoming the hydrated 

 sesqui-Dxide of iron whose symbol is 2(Fe* O^H^O, ordinarily 

 known as iron-rust. 



It is a familiar fact that bright iron or steel may, under 

 favorable conditions, be kept unprotected free from rust for a 

 considerable time, but that when once the process of rusting 

 commences, the rust specs, as centres of corrosion, rapidly 

 spread until the entire metallic surface becomes covered with 

 a sheet of rust. The chemical explanation of this progressive 

 action when rusting has once commenced is, that during the 

 decomposition by oxidation of the ferrous carbonate to ferric 

 hydrate, the entire amount of carbonic acid is not given off, 



