Page:Proceedings of the Royal Society of London Vol 69.djvu/452

434 form of the primary or large crystal grains, and also indicates the existence of very minute crystals within the boundary lines of the larger crystal grains.

The structure and general orientation of the minute secondary crystals, as seen in section by vertical illumination at the higher magnification of 360 diameters, are illustrated on figs. 3 and 4.

The larger or primary crystal grains were observed to consist of irregular polygons of varying size, the etched indications of the facet junctions being often clearly and sharply defined.

The minute or secondary crystals (whose inter-crystalline junctions were also clearly seen) were in the mass observed to be in varied positions of orientation, but the orientation was generally identical, or on the same plane, within the area of each larger crystal grain.

The general orientation of the smaller crystals varied, however, in each separate larger crystal, and the consequent varied reflection of the light has given the face of the micro-section, as a whole, the appearance of lighter or darker areas in the photo-micrographs. This effect is merely optical.

In some portions of the mass there were observed minute triangular crystals; these ap'pear, however, to be only developments resulting from the cutting of certain crystals in section.

The general micro-crystalline structure of platinum was observed to be allotriomorphic in character and derived from a system of inter- fering cubes and octahedra, the cubic and hexagonal form being frequently noticeable.

The size of the large crystal grains varied from about O002 inch to 0'04 inch in size, and the smaller crystals ranged from about 0'0002 inch to about 0*007 inch.

There were indications that the smaller or secondary crystals were each built up of even more minute crystalline ramifications.

The crystalline structure of platinum appears to generally resemble that of gold and silver.

The descriptive words " primary " and " secondary " crystals are only used in relation to size, and they are not intended to convey the idea of distinctive times of formation during solidification.

In connection with the present investigation reference may be made to the valuable researches of Professor Liversidge on " The Crystal- line Structure of Gold and Platinum Nuggets."* On comparison there appears to be considerable identity between the crystalline structure of natural platinum nuggets and that of the platinum ingot


 * ' Journal, Royal Society of New South Wales,' 1897, vol. 31, p.' 70, Plate 9.