of Bronzes from Machu Picchu, Peru. 561 



theoretical hypotheses which are, themselves, in an imperfect 

 state of development with respect to the ultimate constitution 

 of solid material. Without considering the nature and 

 arrangement of the molecules or molecular aggregates within 

 the individual grains, we may freely observe, apart from any 

 particular theory of deformation, that severe deformation pro- 

 duces numerous subdivisions or dislocations within the original 

 grain which are unstable in form and re-orientate, forming 

 new individuals, on annealing. Relations between surface 

 tension and grain size seem most adaptable in explaining the 

 characteristic alterations of grain on annealing, although 

 Rosenhain (19) dissents from this view and presents his own 

 ideas of grain growth. 



Miceographic Characteristics of Deformation. 



Micrographic methods are not particularly serviceable in 

 detecting the strained condition within a body of metal. The 

 moderate stresses which serve to develop slip bands upon a pre- 

 viously polished surface leave the metal with sensibly unaltered 

 internal structural characteristics. The metal may be strained 

 sufficiently to show a distinct increase in hardness by direct 

 test and to recrystallize on anneal, without visible microscopic 

 alteration of structure. As deformation increases, the normally 

 rectilinear junctions between some of the twinned grains become 

 visibly curved and, later, the etching contrast between neigh- 

 boring grains decreases along with the appearance of lines of 

 deformation. The latter are straight or curved lines which have 

 etched selectively at the expense of the surrounding material 

 and indicate a profound alteration of the metal. They are 

 typical in appearance and always occur when the deformation 

 has been severe. Their arrangement is not such as to indicate 

 the order of fragmental resolution or destruction of grain by 

 deformation. They merely constitute the principal etching 

 characteristics of highly deformed bronze, brass, and other 

 alloys, and can only be interpreted in a qualitative sense. Ordi- 

 narily, the sectional area of a specimen must be reduced at least 

 15 or 20 per cent by rolling in order to bring about a distinct 

 development of these lines all over the (small) section. The 

 nature of the deformational process cannot be disregarded in 

 seeking to characterize the appearance of the lines or the point 

 at which they first appear. Thus, the same reduction by roll- 

 ing, drawing, or hammering would not develop precisely the 

 same etching characteristics and, moreover, the mechanical 



