Page:Proceedings of the Royal Society of London Vol 69.djvu/451

Rh starting from the very first principles ; and it is from such study that this purely mechanical account of the physical evidence has been obtained.

The crystalline structure of platinum does not appear to have been studied, although it forms an interesting subject for investigation.

A small ingot of pure platinum was obtained for the experiments. A section was cut therefrom and machined to T 5 ff inch square and y 1 ^ inch in thickness. The section was then carefully polished and etched in aqua regia of the following strength :

4 parts of hydrochloric acid (sp. gr. 1 '2). 1 part of nitric acid (sp. gr. 1'42).

When the polished micro-section was immersed in the aqua-regia solution in the cold, no solvent action occurred, although the metal was exposed for a considerable time in the reagent.

The temperature of the etching solution was then gradually raised to boiling point, at which it was maintained for 15 seconds, but it had apparently no visible effect on the platinum, as ascertained by micro- scopical examination, after having previously washed the section with water and pure alcohol.

The etching process was repeated and the metal again exposed for a further period of 30 seconds, but after microscopical examination of the surface of the metal it was found that even this exposure had not satisfactorily developed a structure in the metal.

The etching process was again repeated and the micro-section exposed for another 15 seconds to the boiling aqua-regia solution above described. This developed a better crystalline structure, though not a thoroughly satisfactory one. The same micro-section of platinum was therefore again exposed to the action of boiling aqua regia for a further period of 45 seconds, resulting in the development of a beautiful crystalline structure which manifested not only the large or primary crystal grains but also the secondary or very minute crystalline development which is illustrated on Plate 6.

Fig. 1 shows the micro-crystalline structure of pure platinum as seen in transverse section by oblique illumination at a magnification of 50 diameters.

Fig. 2 shows, at a magnification of 120 diameters, the crystalline structure as indicated in transverse section by the direct or vertical method of illumination. This micro-section illustrates the general