of Edinburgh , Session 1881-82. 
369 
After having passed in review the chief properties of the 
different kinds of iron, the paper goes on to discuss the different 
theories that have been proposed to account for these properties. 
An objection is raised against the different theories which 
consider the iron and carbon as chemically united together, on 
the grounds that if these hypotheses be true, we are then presented 
with an anomaly unknown in any other instance, viz,, that of 
two elements uniting together in all proportions up to a certain 
point, and then suddenly losing this power, and it is very difficult 
to believe that such can be the case, whilst another difficulty is the 
fact of the carbon being capable of changing its condition and 
passing in and out of combination under the action of heat or 
different methods of cooling, in a manner at once extraordinary 
and totally different from anything else with which we are ac- 
quainted in the whole range of chemistry. 
A new hypothesis is then given with regard to the nature of 
the different kinds of iron. 
The carbon is considered to be in a state of solution in the iron, 
and it is shown (by analogy of what takes place in the case of 
a solution of carbon in silver) how if the metal be cooled slowly 
the carbon by preference crystallises in the graphitic form, which 
accounts for the carbon in slowly cooled steel and cast iron being 
chiefly in that condition. But how if the cooling be rapidly 
effected by plunging the metal in water or running it into a cold 
mould (as in chill casting) then the carbon is not as it were given 
the option as to which form of crystallisation it will take, but is 
caused to crystallize in the diamond form, and in this way the 
hardness of steel and chilled cast iron is accounted for by the 
presence of an innumerable quantity of excessively minute diamond 
points disseminated over the whole surface of the hardened metal. 
It is then shown how (supposing this hypothesis to be correct) 
such points of difficulty as the following can be explained, 
namely : — 
1. What constitutes the difference between steel and white cast 
iron, and between white and grey cast irons 1 
2. Why steel requires some time after fusion of the metal to 
become good steel 1 
3. How the hardening of steel takes place 1 
