ON THE CRYSTALLINE STRUCTURE OF METALS. 
289 
of a freshly-squeezed specimen ; the following photographs were taken after the 
specimen had been exposed to 200° C. for the time shown in the table :— 
Number. 
Days. 
Hours 
Fig. 21 . . . 
— 
L7-5 
99 
1 
17-5 
„ 23 . . . 
9 
— 1 
17*5 
„ 24 . . . 
5 
16 
„ 25 . . . 
39 
20 
„ 26 . . . 
39 
20 
Fig. 20 shows the small structure characteristic of freshly-strained lead, with one 
or two larger crystals that have persisted from the original crystallisation. In 
fig. 21 these are seen to have grown considerably, and a general change of pattern 
is observed; the most striking feature is the large skeleton crystal that has 
developed in the lower left-hand corner of the marked area. This skeleton is seen 
to grow and fill in continuously in figs. 22 and 23. Figs. 24 and 25 were taken under 
a different angle of illumination in order to show another large crystal which gave 
signs of vigorous growth. In fig. 24 it is still somewhat skeletal, but in fig. 25, 
while it has become much more consolidated, all its outlying arms have disappeared, 
having been absorbed by a well-defined crystal, part of which is seen as a dark arm 
in fig. 25. Fig. 26 is a photograph of the sjjecimen at the same stage as fig. 25, but 
at a lower magnification (8 diameters), and so illuminated as to bring the new crystal, 
whose dark arm is seen in fig. 25, into brightness. This new crystal is seen to be 
enormously larger than its neighbours, and from its position relatively to the marked 
area it can be recognised as the same crystal whose early stages are seen in figs. 22 
and 23. This large crystal is an excellent example of what may be called an aggressive 
individual crystal. Another example, also at 8 diameters, is shown in fig. 27, Plate 10. 
In both figures the edge of the specimen can be seen, and the photographs illustrate 
the observation that the largest crystals are generally near the edges of the speci¬ 
men. It should be added that these large crystals are not mere surface layers, but 
extend well into and across the thickness of the specimen, and can be readily identified 
on the opposite surface when, as in this case, the specimen is a plate about one-eighth 
of an inch thick. 
The occurrence of such large crystals in the annealed metal is apparently in no way 
dependent upon the size of the crystals in the original state before straining; the 
large crystal in fig. 27 was developed in a specimen whose original crystals were 
very small. 
A remarkable feature, well illustrated by the photographs (figs. 26 and 27) of these 
large crystals, is the frequent occurrence of twin crystals, both as inclusions in the 
body of the crystal and at the boundaries. In fig. 26 three distinct sets of straight 
VOL. CXCV.— A. 2 P 
