Prof. It. Bunsen on Rhodium, 257 



mobility, and in the readiness with which the small globules 

 coalesce to larger ones. When the chloride has taken up as 

 much of the oxide as it can, the layer of oxide reappears ; but by 

 being again sprinkled with sal-ammoniac this disappears. The 

 fused zinc strewed with sal-ammoniac has another property in 

 common with mercury — that of instantaneously adhering to 

 many other metals, and, in case they form alloys with zinc, of 

 rapidly dissolving them. When the sal-ammoniac is strewed on 

 the surface, the chloride of zinc bubbles up, in consequence of the 

 ammonia and hydrogen liberated by the zinc. Several oxides 

 and chlorides, especially those of the platinum-metals, oncoming 

 into this layer of chloride, which is kept in rapid agitation by 

 reducing gases, are reduced and rapidly dissolved by the zinc. 

 On this reaction a very simple process is based of quantitatively 

 separating platinum-metals from all substances which are neither 

 reduced nor dissolved by zinc. If, for instance, osmium-iridium 

 is immersed in the layer of chloride of zinc, in a few minutes it 

 is completely dissolved by the zinc regulus at a temperature not 

 much above that of the fusing-point of zinc, especially if the 

 porcelain crucible, which is being heated by a Bunsen's burner, 

 is kept for a few minutes in rotation. The gangue mixed with 

 the grains of osmium-iridium is left in the basic chloride of zinc. 

 If the regulus, after it has just solidified, be lifted out of the still 

 liquid layer of chloride of zinc, and if together with the contents 

 of the crucible it be rinsed in a porcelain dish with acetic acid 

 just sufficient to dissolve the basic chloride, the gangue can be 

 separated by filtration and quantitatively determined. In this 

 operation the crucible is not in the least attacked ; but if the 

 regulus be not removed immediately after solidifying, the cru- 

 cible is generally broken in cooling, owing to the unequal con- 

 traction of the metal and of the porcelain. When the operation 

 is properly conducted, there is no trace of metallic globules in 

 the slag. In order that the experiment may be made at as low 

 a temperature as possible below the boiling-point of chloride of 

 zinc, the alloy of zinc formed must not be too difficultly fusible, 

 which is attained by having from twenty to thirty parts of zinc 

 to one of the platinum-metals to be expected. 



For the extraction of platinum residues which have been pre- 

 viously exhausted with nitric acid this method is admirably suited. 

 By one fusion for two or three hours all the platinum-metals 

 were extracted to the last trace. The details of the process are 

 as follows : — From 3 to 3*5 kilogrammes of commercial zinc 

 waste are fused in a Hessian crucible holding 2 litres, sal- 

 ammoniac being occasionally added; the residue, weighing 0*4 

 kilog., which has previously been gently ignited with sal-am- 

 moniac, is projected into it, and the mixture kept for two to 



Phil. Mag. S. 4. Vol. 36. No. 243. Oct. 1868. S 



