ON THE CRYSTALLINE STRUCTURE OF METALS. 
287 
The figures given above must therefore be taken as no more than an extremely 
rough indication of the order of magnitude with which we are concerned in these 
questions of growth at ordinary temperatures. 
Taking a specimen of plumbers’ sheet-lead showing fairly large crystals as an 
example of metal in which air-temperature annealing has continued for a long time, 
we next tried the experiment of applying a higher temperature, so as to determine 
whether any further change would occur. Our observations show that the metal in 
this state is very sensitive to moderately high temperatures, three minutes’ exposure 
of ordinary sheet lead to a temperature of 200° C. being sufficient to produce a great 
change in the crystalline pattern. If the specimen be kept at 200° C. for a long time 
there is a continued change, but it becomes very slow, and ultimately a state is 
reached when further change is imperceptible. 
Figs. 14 to 19, Plates 6 and 7, inclusive are a series of micro-photographs of a 
specimen treated in this way. Fig. 14 shows the appearance of a typical specimen of 
sheet-lead, and fig. 15 shows the change produced by 30 minutes at 200° C. Except 
in one or two isolated instances it is very difficult to trace any connection between 
the pattern now seen and the original. Fig. 16 shows the same surface, re-etched 
after a further 30 minutes at 200° C. There has been further change, but not to 
anything like the same extent as in the first half-hour. The change is most marked 
in a large crystal on the left-hand side of the figure ; in fig. 15 it shows a mottled or 
skeleton appearance, and this becomes filled in in fig. 16, while there is a considerable 
increase in the length and thickness of the tv T o tongue-like projections that start on 
the right-hand side of the twin band. 
A further development of this crystal is seen in fig. 17, which was taken after 
48 hours’ further annealing at 200 : C. Here another twin band has become evident, 
and a further filling in and straightening of the boundaries has taken place. This 
process has gone slightly further in fig. 18, taken after four days’ further annealing. 
This photograph may be taken as representing the final state of this specimen, as 
very prolonged annealing produced no further considerable change. This specimen, 
however, showed another interesting feature, which we have often observed in other 
cases. The marked area chosen for photography occupied the centre of the surface 
of the specimen, which measured approximately f inch square by -§• inch thick. 
Somewhat to our annoyance, this marked area did not show by any means the best 
development of crystalline growth. In this case, as in many others, we found that 
the largest and most rapidly growing crystals were formed at or near the edges of 
the specimen. Fig. 19, taken at the same stage as fig. 18, and with the same 
magnification, but outside the marked area, shows the remarkable development of 
crystals in some parts of the specimen. 
In comparing the members of such a series of photographs, one consideration must 
be borne in mind, the great difference produced in the appearance of the surface by 
a minute change in the angle of incidence of the light. In spite of the utmost 
