294 
PROFESSOR J. A. EWING AND MR. W. ROSENHAIN 
If a specimen of such sheet-zinc be exposed to a temperature of 200 C. for about 
half-an-hour, it shows on etching with strong hydrochloric acid a brilliantly crystalline 
texture as illustrated in fig. 36, Plate 13, magnified 8 diameters. The metal is then 
much harder, but at the same time brittle ; when bent it emits a “ cry ” like that o± 
tin, and finally breaks with a crystalline fracture which zig-zags along the cleavage- 
planes of the various crystals through which it passes. 
Many of the phenomena described above as occurring in such metals as lead, tin, 
zinc, and cadmium, have close and well-known analogues in iron and steel. In those 
metals, however, the temperatures involved are much higher, and the experimental 
difficulties are consequently greater; we have consequently been able to make many 
observations on the re-crystallisation of lead which it has not yet been possible to 
make in the case of iron or steel. The question, therefore, suggests itself, how far 
the analogy between the various metals holds true. One view which is held by 
many persons—perhaps unconsciously—may be called the theory of “ corresponding 
temperatures ” ; the central idea of this view is that the properties and behaviour of 
metals are to a great extent a function of their distance from their own melting 
points. Thus a phenomenon seen in lead at 200° C. is strictly analogous to a similar 
phenomenon seen in iron at 800° C. To a certain extent this view is justified, but it 
may easily be carried too far. Thus, while lead at 10° C. is soft and ductile like iron at 
800° C., yet gold at 10° C. is also soft and ductile, while it is further from its melting- 
point than iron at the same temperature. Again, one of the most characteristic 
features of lead is the extreme straightness of its “ slip-bands,” while those of iron 
are characteristically curved and irregular. We have not had an opportunity of 
examining the slip-bands produced by straining red-hot iron, but we have made the 
converse experiment of straining lead at the temperature of liquid air, with the 
result that the slip-bands developed were as characteristically straight and regular 
as ever, thus showing that the straightness of the slip-bands, which is so closely 
associated with softness and ductility, is a characteristic of the metal rather than a 
function of the temperature. In order to thoroughly test the same question in rela¬ 
tion to annealing, a long series of experiments to determine whether or not gradual 
re-crystallisation goes on in strained iron at ordinary temperatures and a similar series 
on lead at much lower temperatures would be required. In the case of glass-hard 
steels it is well known that a certain amount of annealing takes place at 100° C., and 
even at atmospheric temperatures ; but in the case of steel, where a distinct chemical 
change is involved, the nature of the annealing action may differ very considerably 
from that in an approximately pure metal. Certain phenomena in the recovery of 
elasticity after over-strain in both iron and steel (Muir, J., “Recovery of Iron and 
Steel from Over-strain,” ‘ Phil. Trans./ 1899) also point to the fact that changes of 
internal structure may occur in iron at very moderate temperatures ; further evidence 
in the same direction is afforded by the effect produced (Poget, S. R., ‘ Proc. Poy. 
Soc.,’ 1898) on the magnetic properties of soft iron by roasting at moderate tempera- 
