ON THE CRYSTALLINE STRUCTURE OF METALS. 
285 
The first step was to determine the effect of very severe strain on the crystalline 
structure of lead. In such a soft, ductile metal, plastic deformation may be carried to 
so great an extent that the adaptability of the individual crystals to change their 
shape by means of slips on cleavage planes may be insufficient. Careful observation of 
the crystalline structure of a piece of lead under severe compression confirms this 
view. Up to a certain point the crystals are gradually flattened out in proportion to 
the flattening of the whole specimen, but when the “ flow ” becomes considerable it is 
found that the crystals, already very thin and flat, are driven into and through one 
another, this process resulting in a grain or structure which is small, but still entirely 
crystalline. The action is analogous to what occurs in the fracture of a more brittle 
metal, with this difference, that in a more brittle metal, when “ slip ” has gone so far 
as to extend right across a crystal, the new surfaces thereby brought into contact do 
not unite or “ weld,” and fracture results ; in lead the freshly exposed surfaces do weld 
or re-unite under the pressure, a fact which is associated with the possession, on the 
part of the metal, of great ductility. Fig. 6 is a micro-photograph showing the crystal¬ 
line structure of ordinary cast lead magnified 1.2 diameters, while fig. 7 shows the much 
more minute structure of freshly and severely-strained lead magnified 30 diameters. 
In this and in the following experiments with lead, the process of straining was carried 
out by squeezing a cast block of the metal in a compression-testing machine, letting 
it expand laterally until the block, originally about 1 inch high and inch diameter 
was flattened into a plate about fr inch thick. 
In order to investigate the changes in the crystalline structure of such strained 
lead, we adopted the method of taking a series of photographs of a marked area of 
each specimen at intervals of time during which the metal was exposed either to the 
ordinary temperature of the room or was subjected to special thermal treatment. 
Before each photograph was taken the surface was thoroughly re-etched ; our experi¬ 
ments on the annealing of iron had convinced us of the necessity of this proceeding, 
and our observations on lead specimens have confirmed the previous experience. In 
no case did thermal treatment in any way produce a visible change in the surface 
pattern until further etching had been resorted to, and fairly deep etching is required 
to obliterate the old pattern entirely. This applies more particularly to the channels 
which are etched out at the inter-crystalline boundaries—these may often be seen 
forming a network on the newly-formed pattern, but quite independent of the new 
structure. 
The method of etching consisted of alternate applications of concentrated and very 
dilute nitric acid. In some cases, where very deep etching was required, an electrolytic 
method was used. One of the great advantages of dealing with a metal like lead 
arises from the large size of its crystals; by enabling us to use deep etching it allows 
all fine polishing to be dispensed with, and it becomes possible to obtain micro¬ 
photographs at low magnifications, and under oblique light, which exhibit clearly the 
various changes in crystalline structure. 
