22(i 
MR. J. C. W. HU^IFEEY OX THE EFFECTS OF STRAIX 
large ridge ran down the centre of the crystal, and from this others branched awav 
at right angles, giving the appearance of a sort of skeleton. 
In order to see how the growth of the crystals had proceeded throughout the 
casting, the rough sides were smoothed with an ordinary wood plane, and the whole 
casting was subjected to a prolonged etching in dilute nitric acid. The effect of this 
treatment upon the upper surface was to bring out in a very striking manner the 
orientation of tlie different crystals. The surface of each crystal became covered 
with a number of geometrical pits, these pits being similar and similarly situated 
over its area, though varying iii shape and position from one crystal to another. 
The crystals accordingly showed either blight or dark, as the light striking the sides 
of the ])Its was reflected or not into the eye. As the specimen was revolved the 
crystals flashed out one after another from almost black to a brilliant white. Fig. 1 
(Plate 2) is a photograph of this surface in its natural size, and the different shades 
of the various crystals can be clearly seen. The appearances observed after etching 
ai’e precisely similar in kind to those which are already well known as occurring 
when ordinary metallic surfaces are subjected to microscopic examination ; but here 
they are exhibited on an exceptionally large scale. 
The first effect of etching upon the sides of the specimen vns to reveal a number 
of minute crystals all ovei‘ the side surfaces, but as the action proceeded these 
disappeared, and tlie large crystalline structure of the lump could be seen. The 
crystals upon the upper surface of the casting were found to onl}’- descend to a depth 
(ff about j inch, and a second system had grown vertically uj)wards from the bottom 
of the mould to meet them. The sections of the latter parallel to the upper surface 
of the casting were generally of rather smaller area than those upon the upper 
surface. 
As will be seen in fig. 1, some of the crystals do not reflect the light quite equally 
from all parts of their surface, but seem to l)e shaded. The shaded appearance is 
only visible in certain illnminations, the orientation of the crystal appearing quite 
uniform In all others. When examined under the microscope, it was found that the 
edges of the etched pits upon them were not quite parallel in the various parts. 
Fig. 2 shows part of such a crystal magnified 12 diameters, and it can be seen 
that the edges of the pits between the two dark bands running across the centre are 
not quite parallel to those on the portions on either side ; the difference is, however, 
slight. The effect is jjrobably caused by a slight dlsturl)ance whilst the crystal Avas 
forming, one part being moved relatively to another when tlie metal between them 
Avas still molten. Such a disturbance miolit be brought about Ija" external causes, 
O O 
such as a shaking of the mould, or by stresses set up as the metal cooled. The 
difference of orientation is still further brought out by the fact that the acid has 
acted more strongly AAdiere the tAvo portions join than upon the snrrounding area; 
this etching out has produced furroAvs, Avhich appear as the dark bands in fig. 2. 
Snch an action is alAAmys seen betAveen tAvo patches of different orientation, and 
