102 
of the nascent hydrogen into the interior of the iron, which 
hydrogen may subsequently be given off by gentle heat or 
immersion in a liquid, etc. Secondly, an absorption of the 
acid itself, possibly in a very concentrated form by the 
interstices between the fibres or crvstals of the metal. 
«/ 
That it is possible for a liquid to pass into the interior of 
a piece of iron is I think proved by the sweating of the 
cylinders of hydraulic presses, and also the known diffusion 
of gases through iron. The structure of iron as revealed by 
the microscope and the changes it undergoes during manu- 
facture, by Avhich a spongy mass is by hammering aud 
rolling squeezed together, all go to prove that there are 
numerous cavities in iron and steel. 
It will however be said, the acid must act on the walls of 
the cavity and form a salt of iron with liberation of hydro- 
gen. This may go on to a small extent, but in opposition 
to this view we may bring the experiments of Prof. Bequerel 
on solutions separated by a cracked tube {Comptes Rendus, 
LXXVI), where he shows that no precipitate is formed on 
placing a cracked tube filled with nitrate of lead in a solu- 
tion of potassium sulphate within the crack, thus making it 
probable that chemical interchanges do not take place in 
very minute spaces. 
By this theory we may easily explain the decrease in 
toughness after immersion in acid. For toughness implies 
a certain ease of mobility of the particles. When a piece of iron 
is bent the particles of one side are compressed, thus dimi- 
nishing the minute cavities between the fibres, while those 
of the other side are stretched, and the minute cavities 
elongated. Now if we fill these cavities with a liquid 
this mobility of the particles is prevented, for the cavities 
cannot ^low be diminished in size and the compression of 
the one side cannot now take place, consequently the piece 
tears or breaks off just like a piece of frozen rope. 
It will also explain the acid reaction of the moistened 
