101 
The supposition that the absorption of hydrogen is the 
sole cause of the change in the breaking strain, diminution 
in toughness, etc., attendant on the immersion of iron in 
hydrochloric or sulphuric acids, and that there is no ab- 
sorption of these acids into the interior of the iron, does not 
account for the following phenomena : — 
1. — The gain in weight of a piece of iron by immersion 
in hydrochloric acid is less than by immersion in sulphuric 
acid, as is proved by experiments described in my first 
paper on this subject. 
2. — Iron after immersion in hydrochloric acid sooner 
regains its original state than after immersion in sulphuric 
acid. 
3. — 'If acid iron, i.e., iron which has been immersed in 
hydrochloric or sulphuric acids, be steeped in an alkaline 
solution it sooner regains its original state than with 
immersion in water alone. 
4. — Take two pieces of iron alike in size and quality, and 
immerse one in hydrochloric acid and the other in sulphuric 
acid for some hours, then wash them well in water and dry 
them gently, and leave them in a temperate room for some 
hours more. At the end of that time it will be invariably 
found — that the piece which was in hydrochloric acid is 
covered with a dark-brown red oxide of iron, while the 
piece which was in sulphuric acid will be only slightly 
rusted. 
5. — Litmus paper when applied to the moistened fracture 
of acid iron is slightly reddened. 
All the above phenomena have been observed so often 
and so carefully as to leave no doubt of their invariable 
recurrence if the conditions of experiment be only properly 
observed. 
It seems to me the only satisfactory way of explaining 
all the phenomena is to suppose that when a piece of iron 
is immersed in acid two actions go on, viz. ; An absorption 
