ON THE CRYSTALLINE STRUCTURE OF LEAD. 
235 
The method of carrying out these exjDeriments was as follows :—A sjDecimen 
about 2 inches long and 4- inch broad by inch thick, was cut from a single crystal 
and given a certain amount of tensional strain in the small machine mentioned on 
p. 230, re-etched and photographed. It was then cooked for a short time by exposure 
to a certain temperature in a small asbestos-lined wooden oven, heated by an electric 
incandescent light. After this cooking the specimen was re-etched, first in a 
20 per cent, solution of nitric acid to remove all former pits, and then in a 5 per cent, 
solution to produce a new set. A second photograph was then taken, and further 
processes of cooking, re-etching, and photographing were gone through until no 
further changes were visible. The photograj^hs were taken with a 4-inch Ross lens, 
fitted to the front of the camera, diffused daylight being used as an illuminant. The 
spechnen was kept immersed in a weak solution of nitric acid whilst being photo¬ 
graphed, in order to preserve a clean surface. Care was taken to get the illumination 
the same throughout the series, in order that the same patches of new orientation 
could be recognised in the difierent photographs. In all the following illustrations 
(figs. 18 to 32) the direction of the pull was parallel to the long edges of the specimen, 
that is to say from top to bottom of the page. 
Fig. 15 shows a specimen after straining in tension and re-etching, magnified to 
five diameters. The two lines seen running across had been scratched with a sharp 
steel point, and were originally 1 centim. apart, the straining Ijeing carried on until 
they were 1’5 centim. apart; the other marks were for the sake of identification. 
The only change visible before annealing was a small amount of re-crystallisation 
along the scratches, where the material was, of course, subjected to fairly severe local 
strain, and in two places between the lines, where it will be seen the new orientations 
appear as two small dark patches. Fig. 16 shows the same specimen at the same 
magnification after cooking for 5 minutes at 60° C. and re-etching. It will lie seen 
that great changes have taken place; the former patches of new orientation have 
greatly extended, and others have appeared in various parts of the specimen. The 
greatest change is at the bottom left-hand corner, where a large area has re-crystallised. 
Figs. 17, 18, and 19 show the same specimen magnified five diameters after further 
successive cookings of 5 minutes’, 10 minutes’, and 20 minutes’ duration respectively, 
at 60° C. The re-crystallisation has continued until practically the whole of the 
original orientation is altered. In fig. 18 (Plate 5), the scratches have been almost 
entirely etched away, and with them the small local changes which they had 
produced. The stage at fig. 19 seems to be a final one, and further cooking produced 
no further changes. In cases such as this, where the material had been subjected tc 
a uniform strain throughout, the patches of new orientation go right through the 
specimen, both sides showing a very similar pattern. In this specimen tlie final 
formation consisted for the most part of a few large differently oriented areas 
penetrating right through, each to a large extent split up by twin orientations in 
bands and patches, as can be seen in fig. 19. It will be noticed that the area of the 
2 H 2 
