60 
“On the Influence of Acids on Iron and Steel,” by 
William H. Johnson, B.Sc. 
In a paper published in the Proceedings of the Society 
for March 4th, 1873, I mentioned that if a piece of steel 
wire be immersed in hydrochloric or sulphuric acid for ten 
minutes or more, and then well washed with water and 
dried, that on breaking it bubbles were not seen to rise 
through the moisture on the surface of the fracture as was 
the case with iron wire. Subsequent experiments made 
under the microscope with a power of 250 diameters, how- 
ever, show that very small bubbles are given off with great 
rapidity, sometimes from the whole, sometimes from part 
only of the fractured surface. 
This difference in the behaviour of iron and steel is most 
likely connected with the difference of molecular structure. 
Thus the fracture of a steel wire containing say ‘75 per cent 
of carbon, when seen under the microscope presents a toler- 
ably flat surface, composed of innumerable small, sharp 
crystalline points, while that of iron is rough, more or less 
fibrous or mossy, and the fibres do not end in sharp points. 
These fine crystalline points in the steel, as is well known, 
must facilitate the evolution of bubbles ; consequently they 
are very small, rapidly given off*, and hence invisible to the 
naked eye, whilst the absence of these points in iron causes 
the small bubbles to collect into larger ones, which are 
readily seen. 
The less carbon a steel contains the more its fracture will 
resemble iron, so in a steel containing only *21 per cent, of 
carbon, small bubbles may sometimes be seen by the naked 
eye. 
About 5 oz. of iron wire ‘125 in. diameter, after 10 days’ 
immersion in hydrochloric a6id 1‘20 s.g., was well washed in 
water, dried and placed in a glass tube heated to a tempera- 
ture of a little over 100° C. by a sand bath. Each end of 
the tube was connected with a bottle containing nitrate of 
