2 EwiNG, The Structure of Metals. 



the size of a walnut or an egg. These pieces are 

 separate crystals of water, and the whole thing is a 

 grand aggregate of such separate granules, each granule 

 a crystal in itself. Precisely the same thing is true of any 

 piece of metal. It is true not only of cast metal, where 

 one might readily expect it to be true, but it is true 

 also of metal that has been forged or rolled, and what is 

 more remarkable still, it is true even of a piece of iron 

 which has been forged or rolled in the cold state. It is 

 true also of so very different a type of metal as a sheet of 

 lead such as is used for the covering of a roof. A sheet 

 of lead is crushed enormously in the process of rolling ; 

 nevertheless if you take a piece from a roof and examine 

 it, you find that it has the same general character. It is 

 built up of a great aggregate of crystalline granules. To 

 make this point clear I shall show slides which will allow 

 us to trace some of the evidence by which this general 

 deduction has been arrived at. When we polish and 

 etch the surface of a piece of metal, and then examine it 

 under the microscope, we are able to detect the existence 

 of the various grains or granules of which the whole is 

 composed. We can distinguish in the surface a difference 

 of texture between one grain and another, and we can 

 also distinguish clearly the boundaries between neigh- 

 bouring grains. We may sometimes detect the difference 

 between the grains in a metal which has not been 

 polished or etched at all, as, for instance, when a smooth 

 surface is formed by casting metal against a sheet of 

 glass, but it is generally more apparent after the surface 

 has been etched by the action of a dilute acid or some 

 other solvent. 



An example is shown in Fig. I, Plate /, which is a 

 photograph of the surface of a piece of wrought iro.i, 

 etched by dilute nitric acid, and viewed with a magnifica- 



