356 REPORT —1905. 
ductility are tiot qualities which indefinitely survive the operations of hanimering 
and wire-drawing. A piece of soft gold beaten into a thin plate does not remain 
equally soft throughout the process, but spreads with increasing difficulty under 
the hammer. If carelessly beaten it may even develop cracks round its edges. We 
may assume that the artificers in gold very soon discovered that by heating, the 
hardened metal might be restored to its former condition of softness. 
In connection with the study of the micro-metallurgy of iron and steel during 
recent years it has been recognised that heat annealing is, as a rule, associated 
with the growth and development of crystalline grains, and Professor Ewing and 
Mr. Rosenhain have shown that overstrain is often if not invariably associated 
with the deformation of these crystalline grains by slips occurring along one or 
more cleavage planes. This hypothesis, though well supported up to a point by 
microscopic observations on a variety of metals, offers no explanation of the 
natural arrest of malleability or ductility which occurs when the overstrain bas 
reached a point at which the crystalline grains are still, to all appearance only 
slightly deformed. At this stage there is no obvious reason why the slipping of 
the crystalline lamelle should not continue under the stresses which have initiated 
it. But far from this being the case, a relatively great increase of stress produces - 
little or no further yielding till the breaking-point is reached and rupture takes place. - 
The study of the surface effects of polishing, already referred to, had shown 
that the thin surface film retained no trace of crystalline structure; while it also - 
gave the clearest indications that the metal had passed through a liquid condition « 
before settling into the forms prescribed by surface tension. Irom this it was - 
argued that the conditions which prevail at the outer surface might equally pre- 
vail at all inner surfaces where movement had occurred, so that every slip of one : 
crystalline lamella over another would cause a thin film of the metal to pass - 
through the liquid phase to a new and non-crystalline condition. By observations - 
on the effects of beating pure gold foil, it was found that the metal reached its: 
hardest and least plastic condition only when all outward traces of crystalline: 
structure had disappeared. It was also ascertained that this complete destruction: 
of the crystalline lamellae and units could only be accomplished in the layers near’ 
the surface, for the hardened substance produced by tke flowing under the hammer 
appears to encase and protect the crystalline units after they become broken down 
to a certain size. By carefully etching the surface in stages by means of chlorine 
water or cold aqua regia, the successive layers below the surface were disclosed. 
The surface itself was vitreous; beneath this was a layer of minute granules, 
and lower still the distorted and broken-up remains of crystalline lamelle 
and grains were embedded in a vitreous and granular matrix. The vitreovs- 
looking surface layer represents the final stage in the passage from soft to 
hard, from crystalline to amorphous. By heating the beaten foil, its softness 
was restored; and on etching the annealed metal it was found that the 
crystalline structure also was fully restored. Photomicrographs showing these 
appearances are exhibited. ‘These microscopic observations were fully con- 
firmed by finding well-marked thermo-electrical and electro-chemical distinctions 
between the two forms of metal, the hard and soft or the amorphous and the 
crystalline. The determination of a definite transition temperature at which the 
amorphous metal passes into the crystalline metal further confirms the phase 
view of hardening by overstrain and softening by annealing. 
It was subsequently proved that the property of passing from crystalline to - 
amorphous by mechanical flow, and from amorphous to crystalline by heat at a defi- 
nite transition temperature, is a general one which is possessed by all crystalline 
solids which do not decompose at or below their transition temperature. The sig- 
nificance of this fact I venture to think entitles it to more than a passing reference. 
Jt appears to me to mean that the transition from amorphous to crystalline is » 
entitled to take its place with the other great changes of state, solid to liquid, 
liquid to gas, for like these it marks a change in the molecular activity which: 
occurs when a certain temperature is reached. It is entitled to take this place: 
because there is every indication that the change is as general in its nature as the - 
other changes of state. Compare it, for instance, with the allotropic changes with - 
