154 THE ACTION OF SEA-WATER UPON CAST-IRON. 
The rapid action of sea-water upon cast-iron and the commoner 
varieties of wrought-iron is probably due mainly to the local gal- 
vanic action set up between the diffused scales of graphite, films 
Where cast-iron is exposed to the combined action of fresh 
water and of sea-water, as at the mouths of rivers, the action is 
hydrogen. 
The action of acid waters in copper and other mines, and of 
means affording a passage for the electric current. 
. h which recently 
of rain, and he states that it takes place much more rapidly le 
case of castings made in “dry sand” or “loam” than in those dry 
sand,” But I am inclined to think that this difference 8 
due in part, if not wholly, to the formation of a film of ee ms 
oxide of iron by the steam from the moisture: the protective & am 
of such a film has long been known, although it was ch “= ty ot 
