336 ' THE president's address. 



througli tlie spongy surface of rust wliicli covers it. In this way 

 tlie rust acts and re-acts, ever becoming a carrier of atmosplieric 

 oxygen to the innermost parts of the iron, until the whole of it 

 is completely corroded and eaten through. 



All the attempts which have hitherto been made to protect the 

 outer surface of iron from the beginnings of rust, whether by 

 paint, or varnish, or cement, or other substances have, only been 

 partially successful, because these substances never really amal- 

 gamated with or inseparably adhered to the metal on which they 

 were placed. Therefore, the smallest flaw in the covering, either 

 from its scaling off, or wearing off immediately admitted the 

 enemy, atmospheric moisture, to the exposed spot. The mis- 

 chief then began, and rust spread laterally as well as inwards. 



You will now understand how rust works, and how rapidly 

 iron may be, and is, destroyed by it. 



Besides the protoxide, and sesquioxide, both of which cause 

 iron to corrode, there is a third oxide of iron, called magnetic or 

 llach oxide, containing three times 56 parts by weight of iron, and 

 four times 16 parts by weight of oxygen. Some chemists call 

 this ferroso-ferric-oxide ; but this oxide undergoes no change what- 

 ever when acted on by moisture or by atmospheric oxygen, or by 

 acids, or corrosive substances, except such as pure hydro-chloric 

 acid ; nor does any temperature to which it can be exposed in 

 any ordinary use, either decompose it, or produce its further 

 oxidation. 



Professor Barff has discovered that if any iron surface is ex- 

 posed to a high temperature by the action of superheated steam, 

 it will become covered with a film of this magnetic black oxide. 

 The thickness of this film is determined by the degree of tem- 

 perature, and by the length of time to which the object is ex- 

 posed to it. The magnetic or black oxide is harder than the 

 original iron, and adheres to it even more firmly than the particles 

 of iron adhere to each other, so that the iron gains somewhat in 

 mechanical strength, as well as in chemical resistance. By 

 heating the operating chamber to 500 degrees of Fahrenheit, 

 and by exposing the iron surfaces to this temperature for 5 hours, 

 a surface or coating is obtained which will not rust from any in- 

 door moisture, or even a moderate degree of outdoor exposure, and 

 will resist considerable friction from emery powder. But if the 

 oxidizing process be carried on at 1,200 degrees of Fahrenheit, 



