666 



SCIENCE 



[N. S. Vol. XLV. No. 1174 



igneous rocks at several places in the world 

 but not in commercial quantities. Tlie search, 

 for platinum in rocks is therefore not likely to 

 obtain an immediate supply of the metal. 

 Persons searching for platinum ores should re- 

 member, however, that the assay for platinum 

 is difficult and apparently can not be success- 

 fully made by all commercial assayers. 

 Samples of supposed platiniferous ores should 

 therefore be sent only to the most competent 

 assayers. The United States Geological Sur- 

 vey has received several reports of discoveries 

 of rich platinum ore in which, as the reports 

 state, " the platinum could be detected by the 

 ordinary methods of assay." Such statements 

 should be regarded with great caution, for any 

 platinum ore of commercial grade wiU. doubt- 

 less yield traces of platinum if tested by the 

 standard methods employed by competent and 

 reliable assayers. 



The platiniun supplies of the world, except 

 a relatively small quantity, have been obtained 

 from placer deposits, notably from those of 

 Eussia, which have produced about 95 per 

 cent, of the world's output. The largest part 

 of the crude placer platinum now produced in 

 the United States is won by dredges working 

 in California at the west base of the Sierra 

 INevada, in gravels derived from wom-down 

 lodes and concentrated by natural streams. 

 The greater production from this region than 

 from northwestern California and southwest- 

 em Oregon and other places would appear to 

 be due to larger operations rather than to 

 greater or richer deposits. 



All the known placer deposits that contain 

 platinum are near areas of basic igneous rocks, 

 and it would seem that the first step in any 

 search for new deposits of platiniferous gravels 

 is to look for outcrops of peridotite, pyrosenite, 

 dunite and serpentine. When areas of these 

 rocks have been found the gravels of the 

 streams that rise in them should be washed to 

 see whether they contain platinum. Most of 

 the heavy concentrates found in gravels that 

 carry platinum are rich in chromite and 

 olivine. The character of the rock of which 

 the gravels were formed may give a clue to 

 their source. 



Crude platinum as it occurs in placer con- 

 centrates is ordinarily a silvery-white metal, 

 the fresh surface of which could be confused 

 only with that of silver, or possibly with that 

 of iron. It can easily be distinguished from 

 these metals, however, by simple tests, as they 

 are soluble in dilute nitric acid, whereas crude 

 platinum can be dissolved only in concentrated 

 aqua regia, a mixture of three parts of hydro- 

 chloric (muriatic) acid and one part of nitric 

 acid. In some placer deposits the grains are 

 coated with a dark film and somewhat resemble 

 the grains of the dark minerals chromite, mag- 

 netite or iknenite, from which, however, they 

 can be separated by careful panning, as the 

 platinum is heavier than any of those min- 

 erals. 



Platinum wiU not amalgamate with quick- 

 silver alone, but will amalgamate with quick- 

 silver and sodium. If ordinary quicksilver 

 amalgam is used the flakes of platintun float on 

 the surface and can be removed. If sodium 

 amalgam is used the platinum may be sepa- 

 rated from any gold in the amalgam by wash- 

 ing with water until the sodium is converted to 

 sodiiun hydroxide, when the platinum will 

 come out on the surface, provided the amalgam 

 is sufficiently liquid. 



Platinum has a hardness of 4 to 5 and can 

 be scratched with a knife. It is so malleable 

 that it can be hammered into very thin sheets 

 without heating. It is practically infusible, 

 and grains of it can not be melted together, as 

 can particles of gold and lead. 



One test for platinum is relatively simple. 

 The metallic particles to be tested are dis- 

 solved by boiling in concentrated aqua regia 

 and the resulting solution is allowed to dry. 

 The dry residue is dissolved in hydrochloric 

 acid, and the solution is evaporated by boiling 

 until it becomes a thick mass but is not quite 

 dry. This mass is diluted with distilled water 

 and to it are added a few drops of sulphuric 

 acid and of potassium iodide, which cause it to 

 assume a very characteristic wine-red color if 

 it contains considerable platinum or a reddish- 

 pink color if it contains only a small quantity. 

 This test, though it is fairly delicate, wiU not 



