ON THE CRYSTALLINE STRUCTURE OF METALS. 
293 
ance to a distortion or bending of true crystalline plates; such distortion would 
probably arise from differences in the coefficients of expansion of iron and tin 
brought into action by a suitable rate of cooling. Considering the extreme thinness 
of the layer of tin, the amount of distortion might well be purely elastic and 
insufficient to produce slip in the crystals of tin. 
Fig. 32, taken from a different specimen, but to the same scale as fig. 31, illustrates 
the change in the dimensions and arrangements of the tin crystals that can be 
effected by changing the rate of solidification; the crystals in fig. 31 were formed 
in a slowly cooled specimen ; those in fig. 32 by quenching the specimen in water 
while the surface layer of tin was still melted. By means of local quenching and 
re-melting a great variety of patterns can be obtained ; such processes have long 
been in commercial use in the manufacture of what is called “ moiree metcillique” 
It is important to notice that the small crystals of tin which are obtained by 
quenching the melted metal in water do not show any growth when the metal is 
exposed for long periods to temperatures short of the melting-point. Even a 
temperature just short of fusion does not make them grow or re-arrange themselves. 
A solid block of tin may, however, be reduced to a minutely crystalline structure by 
severe compression, and in specimens so treated we have observed re-crystallisation to 
occur at 150° C. 
We also made some experiments on the re-crystallisation of cadmium at moderate 
temperatures. This metal also can be strained by compression until its crystalline 
structure becomes minute through interpenetration of the original larger crystals. 
Fig. 33, Plate 12, is a low-power (12 diameters) photograph of an etched and marked 
area on the surface of a freshly-strained piece of cadmium. Fig. 34 shows the same 
area, re-etched, after 24 hours’ exposure to 200° C. It now shows a well-defined 
crystalline structure. Fig. 35 shows the same area again, after six days’ further 
exposure to 200° C., and a very considerable increase in the size of the crystals is 
visible. In this case, although the gradual growth of some of the crystals is very 
strikingly shown, many of the features that we have observed in the case of lead are 
entirely absent. In the cadmium we can see no invading branches and no aggressive 
individuals, nor does there seem to be any considerable amount of twinning. 
Experiments similar to those just described were also made on specimens of zinc, 
with the result that specimens of zinc strained by compression at ordinary tempera¬ 
tures were found to re-crystallise on exposure to 200° C. Some results obtained with 
sheet-zinc, such as that used for electric batteries, were particularly interesting. It 
is a well known fact that the mechanical properties of zinc are widely different at 
different temperatures, particularly that the metal is soft and ductile at temperatures 
slightly above 100° Cl, and that it is generally worked at that temperature, while it 
is known to become very brittle at and above 200° C. Commercial sheet-zinc, rolled 
at temperatures above 100° Cl, remains fairly soft and flexible at ordinary tempera¬ 
tures, and its crystalline structure is too minute to be seen in specimens etched with¬ 
out previous polishing. 
