ON THE CRYSTALLINE STRUCTURE OF METALS. 
291 
(5) The phenomena of growth of crystals occur in lead only when the metal has 
previously been subjected to severe plastic strain. The structure of a cast specimen 
remains unaltered at temperatures which cause a strained specimen to show rapid 
change. By casting in a mould arranged to cause rapid cooling, specimens of lead 
can be obtained having a minute crystalline structure, whose scale is not very 
different from that of severely crushed lead ; such a specimen was exposed to 200° C. 
for nearly seven days, but no visible change of structure occurred. A piece of this 
specimen was then strained by severe crushing, and on further exposure to 200° C. 
vigorous growth took place. 
(6) The rate at which a lead specimen is cooled from temperatures of 200° C. to 
300° C. down to the ordinary air-temperature has no visible effect on the structure. 
Even “quenching” in liquid air has no visible effect; quenching in water, cooling in 
air, and slow cooling in the oven, were all tried on a number of specimens without 
in any way affecting them. 
(7) Lead is mechanically hardened to a small extent by severe strain, and the 
subsequent effect of annealing in restoring softness is correspondingly small. In one 
of the experiments a specimen of lead was crushed under a given load in the testing- 
machine, and the load was left on until no further creeping occurred. The specimen 
was then annealed and again placed under the same load, when a distinct amount of 
further crushing was seen to take place. 
Some of the experiments described above as having been made with lead were 
extended to certain other metals that lend themselves to similar treatment ; those 
used were tin, cadmium and zinc. 
The crystalline structure of tin is well shown when a surface of a cast ingot of the 
metal is etched with strong hydrochloric acid. These crystals are generally large, 
but a much more striking display is obtained on etching the surface of commercial 
tin-plate. Even before etching, the inter-crystalline boundaries may be seen on the 
surface, where they are marked by fine grooves or channels. The presence of these 
channels is readily accounted for by the method of manufacture, during which these 
plates are drawn out of a bath of melted tin, and allowed to drain. As the plate is 
drawn out, the layer of tin adhering to it crystallises, but any fusible impurity present 
in the tin would remain fluid slightly longer, and, being forced by the crystallising 
tin into the inter-crystalline junctions, the still fluid impurities will drain off, thus 
leaving; a minute channel. 
The appearance of the etched surface of commercial tin-plate is shown in fig. 28, 
Plate 10, which is a photograph at one-half the natural size. In this photograph the 
outlines of the large crystals can be clearly seen, but it also illustrates another and 
peculiar feature of etched tin-plate. In all cases of an etched crystalline metal viewed 
by oblique light we have always observed that, under a given incidence of light, 
certain crystals were bright while others were dark, and that the illumination was 
uniform over the entire area of each crystal. In the etched tin-plate this is not the 
2 P 2 
