ON THE ACTION OF AIR AND WATER UPON IRON. 229 
if the proportion of combined air be the same at all depths, it 
seems probable the corrosion will slightly increase with the 
depth ; but the matter is now under experiment. 
162. The second question, namely, at what temperature the 
corrosive action of water on iron is a maximum, has with each 
(sea and fresh water) two conditions, namely, when the water 
is combined with air, and when it is freed from it by boiling, or 
otherwise. 
163. In the first case, namely, in water and combined air, I 
find corrosion proceeds fastest in fresh water at temperatures 
varying between 175° and 190° Fahr. 
164. I find, further, that the rapidity of corrosion is in the 
direct ratio of the volume of combined air at any given tempera- 
ture; and lastly, I have found, on slowly heating water holding 
air in combination from 60° Fahr. up to 212° Fahr., that the air 
is evolved most freely, and in greatest volume, at 190° to 195° 
Fahr.; hence we at once perceive that the reason why the 
maximum corrosion of fresh water with combined air is between 
175° Fahr., and 190° Fahr., arises from this being the point at 
which the attraction of the water for the air is destroyed, or 
nearly so, and hence the latter left free to combine with the 
metal. 
165. If fresh water be deprived of all combined air, its cor- 
rosive action on iron ceases tm foto in a close vessel; nor does 
corrosion commence at a boiling temperature ; but if the vessel 
be open and very shallow, the heat even of ebullition does not 
prevent the absorption of air; and oxidation, once commenced, 
goes on even more rapidly than at a lower temperature. 
166. Information is yet wanting as to the temperature of 
maximum corrosion of sea water, a question of greater intricacy 
and importance with reference to marine boilers, and now in 
course of experiment. 
167. As regards the third question, namely, the relation be- 
tween the degree of saltness of sea water and its corrosive power 
for iron at common temperatures, I find that in sea water, de- 
prived of combined air, acting on iron in an open vessel, and 
having therefore to originate its corrosion by air drawn from the 
atmosphere, the corrosive power is inversely as the density of 
the solution, or the amount of its saline contents. 
__ 168. And in the case of sea water holding air in combination, 
the corrosive power is compounded of the direct ratio of the 
volume of combined air, and of some function of the amount of 
saline contents. 
169. The tendency of dissolved salts to prevent the absorp- 
tion of air by water, which is such, that a saturated solution of 
