‘284 
PROFESSOR J. A. EWING AND MR. AT. ROSENHAIN 
Further evidence of the crystalline structure of sheet-lead is found when a slightly 
higher magnification (100 diameters) is applied. Many of the crystals are then seen 
to be marked with geometrical figures, most frequently hexagons, but sometimes 
triangles and rectangles. These figures are similar, and similarly oriented, over the 
entire area of any one crystal, but they are generally different in shape and orienta¬ 
tion on adjacent crystals. In this respect they resemble the geometrical etched pits 
in iron and geometrical air-bubbles in cadmium described in our previous paper cited 
above. Their occurrence in cast lead that lias been exposed to dilute nitric acid has 
been described by Professor Mires and Mr. A. Dick in the £ Journal of the Mineral- 
ological Society,’ April, 1899. Professor Miees measured their crystallographic 
angles, but does not connect them definitely with the crystallisation of the solid metal 
—and Mr. Dick’s view is that they are promiscuously deposited crystals due to 
electrolytic transfer of lead from the upper to the lower surface of the specimen. 
Careful microscopic examination shows that in some instances this is the case 
in sheet lead. Some of the geometrical figures can be seen to be the outlines of 
deposited, or at all events, projecting crystals ; but the remarkable fact remains that 
even here these crystals are deposited with their elements in the same orientation 
as those of the crystal upon which they are being deposited. In many cases, how¬ 
ever, geometrical markings are found which form the outlines of pits instead of pro¬ 
jections. Sometimes these pits occur on the same grain as the deposited crystals, and 
both then have similar outline and orientation. 
Fig. 5, Plate 4, is a photograph of such geometrical markings on etched sheet- 
lead, seen under oblique light and magnified 100 diameters. 
The observations here described form a full demonstration of the marked crystalline 
structure of sheet-lead, but the origin of this structure is not immediately apparent. 
Sheet-lead is produced by rolling out cast ingots of the metal without the aid of heat. 
In the sheets we have, therefore, metal which has been subjected to a very great amount 
of plastic deformation. The view of plastic strain enunciated in our former paper 
(‘Phil. Trans.,’ 1900, Series A, vol. 193, pp. 353-375) would lead us to suppose that 
the structure of such violently deformed metal would still be entirely crystalline, but 
its crystals would show peculiarities of shape corresponding with the nature of the 
plastic deformation imposed upon the mass of the metal. Thus, in a specimen which 
had been rolled from a solid lump into a sheet, we should exjiect to find the crystals 
thin and flat, but of considerable area parallel to the surface of the sheet. Sheet- 
lead, however, shows no such feature ; on a transverse section of the sheet no 
flattening of the crystals is seen This led us to suppose that some process of 
annealing or re-crystallisation had been at work in the metal subsequent to its manu¬ 
facture, and, as the lead had not been subjected to any thermal treatment, it seemed 
that re-crystallisation must have occurred in it at ordinary atmospheric temperatures. 
We therefore directed our experiments to test this supposition, and generally to 
investigate the phenomena of annealing or re-crystallisation in lead. 
