ON THE CHEMICAL NATURE OF ALLOYS. 181 



Mylius and Fromm 1 have made an interesting series of 

 experiments on the formation of alloys by precipitation out 

 of aqueous solution. They find that when a plate of zinc, 

 cadmium, or tin is introduced into a very dilute solution of 

 a salt of a less positive metal, the dark fiocculent precipitate 

 which is formed on the surface of the plate contains not 

 only the metal which was in solution, but also a considerable 

 percentage of the more positive metal of which the plate is 

 made. Thus zinc, introduced into a solution of copper sul- 

 phate, causes a precipitate which may contain as much as 

 50 per cent, of zinc, and which, when compressed and bur- 

 nished, is evidently brass. Their explanation of the pheno- 

 menon is that at a spot where the copper has been electro- 

 lytically deposited and wholly removed from the liquid, it 

 finds itself surrounded by a solution of zinc sulphate, so 

 that, with the neighbouring zinc, a couple consisting of 

 zinc, zinc sulphate, and copper is formed. Consequently 

 a thin layer of zinc will be deposited on the copper. This 

 would at once stop the action but for the diffusion of the 

 zinc into the copper as an alloy. The surface of zinc being 

 thus converted into one of alloy, more zinc will be deposited 

 until the copper is saturated with zinc, that is, until the 

 electromotive force of the couple zinc, zinc sulphate, alloy 

 is zero. Laurie's work would lead us to expect that the 

 electromotive force would vanish when the alloy had a 

 definite atomic composition, and Mylius and Fromm found 

 that this was sometimes, but not always, the case. The pre- 

 cipitates of zinc-silver, zinc-copper, and cadmium-silver do 

 not appear to have a definite composition, but cadmium pre- 

 cipitates out of a 1 per cent, solution of copper sulphate a body 

 whose composition corresponds pretty nearly to the formula 

 Cu.Cd. They found that cadmium in a dilute solution of gold 

 trichloride forms an alloy which has exactly the composition 

 AuCcl,, and they remark on the extreme readiness with 

 which gold and cadmium combine. Perhaps the most inter- 

 esting alloy formed by them is the body Cu 3 Sn, which grows 

 in a crystalline form on a plate of tin when immersed in dilute 



1 Berichte d. d. chem. Gesellschaft, xxvii., 630. 



