4 EwiNG, The Structure of Metals. 



They do possess what is really the essential characteristic 

 of a crystal, that it is composed of an enormous number 

 of elementary pieces, which we may call brickbats, in 

 perfectly regular tactical formation. The essential thing 

 about a crystal is, that in its own interior structure it 

 should be composed of perfectly uniform pieces, which 

 are placed together in uniform tactical formation, 

 a formation to which a well drilled body of troops, 

 or the flowers of an early Victorian wall paper, form a 

 somewhat rough approximation. When we come to etch 

 the grain we can detect very clear traces of such a regular 

 tactical formation on the part of the elementary brickbats 

 of which it is composed. Here {Fig. 3, Plate I) for 

 instance, is a photograph showing what is observed in the 

 microscope when the deeply etched surface of a particular 

 metal is examined under a lens of high magnifying power. 

 You can see quite plainly the traces of geometrical forma- 

 tion in the regular tactical grouping of the elementary 

 plates or brickbats. When a structure of this kind is 

 under the microscope, and is illuminated by light coming 

 obliquely from one side, any one grain will reflect 

 more or less light up to the lens, according to 

 particular way in which its brickbats lie. Hence we find 

 wide differences of brightness between grain and grain, 

 but over any one grain the general brightness is uniform 

 (as in Fig: 2, Plate II), because all the brickbats in any 

 one grain face exactly the same way. 



An even more striking example of the same thing is 

 shown in Fig. 4, Plate II. Here the whole visible field is a 

 portion of one grain of a steel containing four per cent, of 

 silicon, which was studied by Mr. J. E. Stead. It lends 

 itself particularly well to the exhibition of this structure. 

 Note that the brickbats when etched do not appear to 

 be of the same size ; but, of course, the microscope does 



