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 oxychloride 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 w^ere immersed in acid for differ- 
