May 18, 1899] 
NATURE 67 
winters ; in’ mild mid-winters the greatest variations of temper- 
ature usually occur in January. After a very mild winter, a 
warm summer is more probable than after a winter which is 
only moderately mild. Dr. Hellmann pleads for synoptic charts 
for the whole globe—at least for short intervals, if longer periods 
cannot be undertaken. ; 
In the Journal of Botany for April and May, Mr. A. Lister 
describes and figures some new or interesting species of Mycet- 
ozoa; Mr. E. A. N. Arber discusses the relationship to one 
another of the various forms of indefinite inflorescence ; Mr. A. 
Gepp records the detection in Britain of a genus of Sapro- 
legneous fungi, Apedachlya ; Mr. G. S. West continues his ac- 
count of the alga-flora of Cambridgeshire ; Mr. F. S. Williams, 
his critical notes on species of Cerxastzum ; and Mr. H. C. Hart, 
his account of a botanical excursion in Donegal. 
SOCIETIES AND ACADEMIES. 
Lonpon. 
Royal Society, March 16.—‘‘ Experiments in Micro-metal- 
lurgy :—Effects of Strain. Preliminary Notice.” By Prof. 
Ewing, F.R.S., and Walter Rosenhain, 1851 Exhibition 
Research Scholar, Melbourne University. _ 
Much information has been obtained regarding the structure 
of metals by the methods of microscopic examination initiated 
by Sorby and successfully pursued by Andrews, Arnold, Charpy, 
Martens, Osmond, Roberts-Austen, Stead, and others. When 
a highly polished surface of metal is lightly etched and examined 
under the microscope, it reveals a structure which shows that 
the metal is made up in general of irregularly shaped grains with 
well-defined bounding surfaces. The exposed face of each grain 
has been found to consist of a multitude of crystal facets with a 
definite orientation. Seen under oblique illumination, these 
facets exhibit themselves by reflecting the light in a uniform 
manner over each single grain, but in very various manners over 
different grains, and, by changing the angle of incidence of the 
light, one or another grain is made to flash out comparatively 
brightly over its whole exposed surface, while others become dark. 
The grains appear to be produced by crystallisation proceed- 
ing, more or less simultaneously, from as many centres or nuclei 
as there are grains, and the irregular more or less polygonal 
boundaries which are seen on a polished and etched surface 
result from the meeting of these crystal growths. The grains 
are, in fact, crystals, except that each of their bounding surfaces 
is casually determined by the meeting of one growth with 
another. f 
The experiments, of which this is a preliminary account, have 
been directed ‘to examine the behaviour of the crystalline grains 
when the metal is subjected to strain. 
For this purpose we have watched a polished surface under 
the microscope while the metal was gradually extended until it 
broke. By arranging a small straining machine on the stage 
of the microscope, we have been able to keep under continuous 
observation a particular group of crystalline grains while the 
piece was being stretched, and have obtained series of photo- 
graphs showing the same group at various stages in the process. 
Strips of annealed sheet iron, sheet copper, and other metals 
have been examined in this way. We have also observed the 
effects of strain on the polished surfaces of bars in a 50-ton 
testing machine by means of a microscope hung from the bar 
itself, and have further observed the effects of compression and 
of torsion. . 
’ When a piece of iron or other metal exhibiting the usual 
granular structure is stretched beyond its elastic limit, a remark- 
able change occurs in the appearance of the polished and etched ; 
surface, as seen by the usual method of ‘* vertical” illumination. 
A number of sharp ‘black lines appear on the faces of the 
crystalline grains: at first they appear on a few grains only, 
and as the straining is continued they appear on more and more 
grains On each grain they are more or less straight and parallel, 
but their directions are different on different grains. At first, 
just as the yield-point of the material is passed, the few lines 
which can be seen are for the most part transverse to the 
direction of the pull. As the stretch becomes greater oblique 
systems of lines on other grains come into view. 
The photograph, Fig. 1, taken from a strip of transformer 
plate (rolled from Swedish iron and annealed after rolling), gives 
a characteristic view of these lines as they appear after a 
moderate amount of permanent stretching, but long before the 
iron has reached its breaking limit. 
NO. 1542, VOL. 60] 
The appearance of each grain is so like that of a crevassed 
glacier, that these dark lines might readily be taken for cracks, 
The real character of the lines is apparent when the crystal- 
line constitution of each grain is considered.. They are not 
cracks, but s/¢ps along planes of cleavage or gliding planes. ; 
Fig. 2 is intended to represent a section through the upper 
part of two contiguous surface grains, having cleavage or gliding 
planes as indicated by the cross-hatching, AB being a portion of 
the polished surface. When the metal is pulled beyond its 
elastic limit, in the direction of the line a B, yielding takes place 
Fic. 1.—Soft sheet iron strained by tension. 400 diameters. 
by finite amounts of slips occurring at a limited number of 
places in the manner shown at a, 4, c, d, e (Fig. 3). This slip 
exposes short portions of inclined surfaces, and when viewed 
under normally incident light, these surfaces appear black be- 
cause they return no light to the microscope. They are con- 
sequently seen as dark lines or narrow bands, extending over 
the polished surface in directions which depend on the inter- 
section of the polished surface with the surfaces of slip. 
We have proved the correctness of this view by examining 
these bands under oblique light.. When the light is incident at 
Fig.2. Before straining. 
only a small angle to the polished surface, the surface appears 
for the most part dark; but here and there a system of the 
parallel bands shines out brilliantly in consequence of the short 
cleavage or gliding surfaces which constitute the bands having 
the proper inclination for reflecting the light into the microscope. 
Rotation of the stage to which the strained specimen is fixed 
makes the bands on one or another of the grains flash out 
successively, with kaleidoscopic effect. In what follows we 
shall speak of these lines as slip-bands. Fig. 1, through a 
mixed illumination, shows some of the slip-bands bright and 
some dark. 
Fig.3. After straining, 
When the metal is much strained a second system of bands 
appears on some of the grains, crossing the first system at an 
angle, and in some cases showing little steps where the lines 
cross. These bands are clearly due to slips occurring in a 
second set of cleavage or gliding surfaces. Occasionally a third 
system of bands may be seen. 
When the experiment is made with a polished but unetched 
specimen the slip-bands appear equally well. The boundaries 
of the grains are invisible before straining; but they can be 
distinguished as the strain proceeds, for the slip-bands form a 
cross-hatching which serves to mark out the surface of each 
grain. 
