75 



one hour or more in either acid is somewhat darker in 

 colour than before. After several hours the fracture may 

 be black in the centre and more or less crystalline in 

 appearance. 



3. Pieces of iron or steel heated in a confined space after 

 immersion in acid become slightly rusted. If air has free 

 access during the application of heat, this is not the case. 



It thus appears that heat expels the dilute acid from the 

 interior of the iron, which if not carried away with sufficient 

 rapidity by the surrounding air attacks the surface of the 

 iron, forming an oxide or oxy chloride of iron. 



Sometimes instead of a uniform coating of rust the iron 

 is simply spotted. The acid will in some cases, after lapse 

 of time, find its way to the surface of the iron and spot it 

 with rust, even without the application of heat ; this is par- 

 ticularly the case with iron which has been soaked in 

 sulphuric acid. 



It is this power which iron possesses of absorbing acid 

 and afterwards giving it off, which accounts for the difficulty 

 hitherto experienced of coating iron with copper, tin, or any 

 other metal in acid solutions. For the acid on coming to 

 the surface of the iron is unable to make its way through 

 the impervious coating of metal, and consequently com- 

 bining with the iron at the surface, forces the copper or tin 

 off. 



4. The universal effect of acid on iron and steel is to 

 decrease its toughness. This brittleness is most marked 

 with steel. Sometimes a coil of steel wire after immersion 

 in acid will break if allowed to fall on the ground. And I 

 have seen hardened steel and steel containing a large per- 

 centage of carbon fly in pieces as soon as it was immersed 

 in acid without being touched at all. 



II. — Effect on the Weight. 

 Pieces of iron and steel were immersed in acid for differ- 



