ACTION OF WATER ON IRON. 273 



" A cylinder of iron, filed bright, is preserved for a long time 

 from all alteration when plunged into a solution of pure potass 

 diluted with one thousand times its weight of water ; but if the 

 solution be left in contact with the air, the alkali, absorbing 

 carbonic acid, by degrees loses its preservative power. 



" When the water contains yf^ of its volume of a saturated 

 solution of carbonate of soda, it forms conical concretions of 

 oxide," &c. 



What is more remarkable in this mode of alteration is, that 

 all points of the surface of the metal are not equally attacked ; 

 the action commences where there are breaks of continuity, or 

 where foreign bodies are deposited, which constitute, by their 

 contact with the iron and the liquid, a voltaic couple : all the 

 rest of the surface pi-eserves its metallic lustre. 



"A saturated solution of sea salt preserved from contact of 

 air only produces some excrescences of oxide of iron ; while, if 

 exposed to air, oxidation goes on as usual. 



"When this solution is saturated with carbonate of soda it 

 possesses the property of preserving iron from all alteration, 

 even though exposed to air, but it loses it when the solution is 

 diluted. 



" It might be presumed that this difference arose from the 

 saturated solutions containing less air than the dilute, but this 

 cannot be so, since M. Payen has proved that the proportions 

 of alkaline bases capable of arresting all oxidation elimhiate but 

 a very small portion of the air contained in the water. 



" M. Payen has determined the proportions of sea salt and 

 sub-cai'bonate of soda which accelerate the formation of tuber- 

 cles (or local corrosion) most. A solution of these two salts, 

 diluted with 75 times its volume of Seine water, produced, 

 in less than a minute, on cast and wrought iron, a commence- 

 ment of oxidation, indicated by light green points, which, in 

 less than ten minutes, formed visible projections." 



The effect is increased by applying to the surface a fragment 

 of charcoal, in which case a voltaic circle is formed ; — " Hence," 

 says the Report, " in similar circumstances, cast iron will alter 

 more rapidly than pure iron." 



We see, then, that solutions which have a feeble alkaline re- 

 action possess the property, in presence of air and sea salt, of 

 producing on cast and wrought iron, which they moisten, local 

 concretions which preserve the remainder of the surface from all 

 change ; and that these effects vary according to the proportion 

 of the different salts, breaks of continuity, and the foreign bodies 

 adherent to the surface of the metals. 



" M. Payen thinks that the concretions formed in the pipes 



