Micro-crystalline Structure of Platinum. 433 



starting from the very first principles ; and it is from such study that 

 this purely mechanical account of the physical evidence has been 

 obtained. 



" Micro-crystalline Structure of Platinum." By THOMAS ANDREWS, 

 F.R.S., F.C.S. Received October 26, Read November 28, 



1901. 



[ PLATE 6.] 



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 



