88 SCIENCE PROGRESS 



microscopical method is no longer confined to investigators 

 in the fields of inorganic and physical chemistry but is becoming 

 general among manufacturers and users of metals, examination 

 by means of the microscope now forming an essential part of 

 the routine in a large and increasing number of metallurgical 

 and engineering works. 



The structure of solid metals is, in the main, crystalline. Of 

 cast and slowly cooled or annealed metals this is probably 

 strictly true, whilst rolled, drawn or otherwise cold-worked 

 metals are built up of material which is only in part crystalline 

 and in part glassy or amorphous. This difference of structure 

 gives rise to important differences of properties between the two 

 materials and it will be convenient to consider metals and alloys 

 in the thoroughly crystalline state before passing on to the 

 modifications brought about by mechanical work. 



Technically, metals (using the term in its widest sense, to 

 include alloys) may be divided into two classes, from the point 

 of view of structure, namely, those which are homogeneous 

 throughout and those which are composed of two or more 

 distinct crystalline constituents. The first class includes the 

 pure metallic elements and also a much more numerous group of 

 alloys, whilst the second class includes all other alloys. The 

 manner in which crystallisation is effected in all members of the 

 first class is essentially the same and a description of the 

 process may serve as an introduction to the general problem. 



The passage of a metal from the liquid to the solid state, like 

 that of any other crystalline substance, does not take place 

 simultaneously throughout the mass but begins at certain 

 nuclei, the number and distribution of these depending on the 

 nature of the substance and on the conditions of cooling. The 

 number of nuclei is greater when the liquid is cooled con- 

 siderably below its freezing-point before solidification begins 

 than when undercooling is reduced to a minimum. It has been 

 suggested that the number is also dependent on the degree of 

 heating to which the liquid has been previously subjected but 

 there does not seem to be experimental justification for such a 

 view, which is one also that it is difficult to accept on theoretical 

 grounds. The chief determining factor is certainly the degree 

 of undercooling. 



The nuclei having once appeared, the further deposition of 

 solid matter takes place around them as centres ; not, however, 



