100 
Dry Carbonic Acid and Oxygen... No oxidation. 
' Oxidation most 
Damp Oxygen and Carbonic Acid 
rapid, a few 
hours being sufficient. The 
blade assumed a dark green 
colour, which then turned 
brown ochre. 
Dry Oxygen and Ammonia No oxidation. 
Damp „ „ No oxidation. 
The above results prove that under the conditions 
described, pure and dry oxygen does not determine the oxida- 
tion of iron, that moist oxygen has only feeble action; dry 
or moist pure carbonic acid has no action, but that moist 
oxygen containing traces of carbonic acid acts most rapidly 
on iron, giving rise to protoxide of iron, then to carbonate 
of the same oxide, and last to a mixture of saline oxide and 
hydrate of the sesquioxide of iron. 
These facts tend to show that carbonic acid is the agent 
which determines the oxidation of iron, and justifies me in 
assuming that it is the presence of carbonic acid in the 
atmosphere, and not its oxygen or its aqueous vapour, 
which determines the oxidation of iron in common air. 
Although this statement may be objected to at first sight, 
on the ground of the small amount of carbonic acid gas 
existing in the atmosphere, still we must bear in mind that 
a piece of iron, when exposed to atmospheric influences, 
comes in contact with large quantities of carbonic acid 
during 24 hours. 
These results appeared to me so interesting that I decided 
to institute several series of experiments. 
When perfectly clean blades of the best quality of com- 
mercial iron are placed in ordinary Manchester water they 
rust with great facility, but if the water is previously well 
boiled and deprived of oxygen and carbonic acid, they will 
not rust for several weeks. Again, if a blade of the same 
metal is half immersed in a bottle containing equal volumes 
of pure distilled water and oxygen, that portion dipping in 
the water becomes rapidly covered with tire hydrate of the 
peroxide of iron, whilst the upper part of the blade remains 
for weeks unoxidized; but if a blade be placed in a mixture of 
