258 Prof. R. Buusen on Rhodium. 



three hours at a temperature not much above the melting- 

 point of the alloy to be formed, while from time to time, as 

 the mass becomes too viscous, some dry sal-ammoniac is thrown 

 upon it. The contents of the cooled crucible consist of three 

 layers : the higher one, which is readily removed by the blow 

 of a hammer, contains no platinum-metals; the second one 

 (which is inconsiderable in quantity) contains a few porous diffi- 

 cultly fusible crumbs of' an alloy of zinc and platinum-metals 

 imbedded in the slag of chloride of zinc; the bottom one con- 

 sists of what is often a very beautifully crystallized regulus. 

 The second layer is coarsely powdered, softened with water, and 

 the metallic crumbs separated by levigation and united with the 

 principal one. To free this as much as possible from all adherent 

 and enclosed impurities, it is once more fused with 0*5 kilog. of 

 zinc, sal-ammoniac being added, granulated in water, and dissolved 

 in hot fuming hydrochloric acid, which takes place with violent 

 disengagement of gas, and scarcely requires half an hour. The 

 chloride of zinc is used for the next operation. The platinum- 

 metals, which settle as a heavy powder like metallic gold, maybe 

 washed by decantation without loss in the course of a few minutes. 

 They are not pure, but contain considerable quantities of lead, 

 copper, and iron, with traces of zinc, arising from the zinc as well 

 as the platinum-metals used. They cannot be separated by 

 means of nitric acid or of aqua regia ; for a considerable portion of 

 the platinum-metals dissolves up, while another is converted into 

 insoluble oxides which are filtered and washed with difficulty. 

 But, curiously enough, the platinum-metals may be almost com- 

 pletely freed from those impurities by digestion with hydrochloric 

 acid. Not merely iron and zinc, but also lead and copper are 

 thereby easily dissolved with disengagement of hydrogen. This is 

 due to the electric current which is set up from the positive 

 metals of the precipitate to the negative platinum-metals through 

 the hydrochloric acid, by which electrolytic hydrogen is libe- 

 rated on the platinum-metals, while the positive metals combine 

 with the liberated chlorine. This reaction may be easily recog- 

 nized by adding a few drops of solution of iridium to hydro- 

 chloric acid in which is a strip of copper, and which is heated 

 to 100° C. The disengagement of hydrogen and solution of 

 the copper commence as soon as it begins to be covered with 

 iridium. The solubility of many metals is increased under 

 these circumstances to a remarkable extent, as indicated by the 

 following experiments made with moderately concentrated hydro- 

 chloric acid. The second column gives the volume of hydrogen 

 which the same surface of the metals of the first column disen- 

 gages alone, in twenty minutes, at a temperature of 100° C 

 The third column gives the quantities of hydrogen which are 



