Inquiry iuto thi Laws of Affinity. p9 ■ 



fold and coppef, in the metallic (late, the influence of thefe two forces, that is to fay, the 

 mutual affinity of the particles of one and the fame metal, and the affinity of one metal for 

 another. 



4. When a piece of copper is plunged in a folution of mercury by the nitric acid, or by 

 the muriatic acid, the copper becomes inftantly white, and the mercury is found to be 

 reduced ; but it has combined with the copper. 



If inftead of a plate of copper, a plate of very clean iron be plunged in the fame folution, 

 feveral hours elapfe before the liquid appears difturbed, and a precipitate is perceived : at 

 length a precipitate is formed ; but with the muriatic folution in particular, this precipitate 

 is partly in the ftate of an oxide, and very probably retains a portion of acid. 



If the affinity of a metal for oxigen were the only caufe which produced the precipita- 

 tion of another metal, the iron ought to a£t with much greater efficacy than the copper ; 

 for it is known to have a much ftronger affinity for oxigen, and neverthelefs its a£lion is 

 flow, difficult and incomplete, while that of copper is inftantaneous. It is feen by the 

 manner in which the Indecompofable acids are retained by the oxide of copper and the 

 oxide of iron, when their combination is expofed to the adlion of heat, that there can only 

 be a very fmall diiFerence between the affinities of thefe metals for the acids. There is 

 confequently no doubt that the affinity of the copper for the mercury, with which it has 

 a£tually combined, muft have greatly contributed to its precipitation in the metallic ftate ; 

 but the mutual affinity of the particles of mercury, muft alone have decided, though with 

 difficulty, in the experiment with iron, the redu£tion of the former metal ; a portion was 

 alfo precipitated in oxide, and probably retained a portion of acid, as the whole would 

 have done, if the affinity of the iron for oxigen had been the fole agent } and the portion 

 precipitated in the metallic ftate did not combine with the iron. 



5. When a folution of filver is precipitated by copper, the precipitate which is found in 

 the metallic ftate is not of pure filver, but a combination of filver with a fmall portion pf 

 copper : it could not take the copper from the plate itfelf which was plunged in the folu- 

 tion ; it muft therefore have precipitated with it out of the folution : the mutual affinity of 

 thefe two metals has decided their difoxigenation. By means of this force two combina- 

 tions are effefted, as happens alfo in feveral other circumftances : one of the acids, with 

 the oxide of copper, the other of filver with a portion of the copper. The a61:ion of th« 

 acid upon the oxide of copper, and that of filver upon copper, are thus put in equilibria. 



6. So likewife when a plate of copper is plunged into a folution of gold, the gold which 

 is precipitated fhews by its higher colour, that it has combined with copper, and the fplu- 

 tion retains but a fmall part of the copper which has been loft by the plate. 



If a plate of iron be put in this folution, the gold which is precipitated feizes, perhaps 

 equally, a part of the iron, or at leaft its precipitation is determined by the affinity of the 

 iron for the gold, at the furface of which this laft combines. For the gilding is a comjbi- 

 nation of the two metals at the furface, by which they are in contad : when the firft ftratum 



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