9 o SCIENCE PROGRESS 



an indication by their form and distribution of the original 

 axial arrangement. 



The growth in length of the axes and consequently the 

 growth in volume of the crystallite is not limited by the develop- 

 ment of external crystal faces but simply by the interference 

 of neighbouring crystallites. The mass of solid metal is ulti- 

 mately composed of polyhedral "grains"; each of these 

 represents the growth about a single primary nucleus, whilst 

 the degree of uniformity of their dimensions is an indication 

 of the regularity of distribution of the nuclei. The grains are 

 the units of crystalline structure in a homogeneous metal or alloy. 

 Their boundaries appear as polygons in a plane section through 

 the solid metal. 



If we consider the common case of molten metal cooling in 

 an ingot mould, it is evident that the temperature of the mass 

 will fall most rapidly at the outer surfaces. The first nuclei 

 therefore make their appearance in contact with the walls ol 

 the mould before the layers at a greater depth are sufficiently 

 undercooled to allow solid matter to separate. In consequence 

 of this distribution of the nuclei, the first crystallites grow 

 inwards from the surface. If the conditions of cooling are 

 uniform, these crystallites are approximately equally spaced 

 and tend to grow as parallel, elongated, more or less prismatic 

 masses recalling to a botanist the form of the " palisade 

 parenchyma " of a leaf. In a small ingot or in one which has 

 cooled with extreme slowness, these parallel crystallites may 

 extend so far inwards as to meet in the middle, whilst in 

 larger ingots or under more usual conditions of cooling they 

 merely form an outer layer, the interior being made up of 

 smaller crystallites without parallel orientation. 



The typical structure of an ingot of pure metal as seen in a 

 transverse section is, then, a number of irregular polygons, of 

 which the outermost are parallel to one another and perpen- 

 dicular to the faces of the ingot, whilst those in the interior are 

 of approximately equal size and are not developed in any chief 

 direction. Naturally, as there is no chemical difference between 

 any one part of the section and any other, the structure is not 

 seen in a section which has merely been cut and polished but 

 in order to reveal it etching with a corrosive agent is necessary. 

 Thus, for example, a surface of copper may be etched with 

 nitric acid. The copper is attacked and its surface is roughened. 



