J 32 Researches refpeBing 



deeper colour, that it has combined with copper, and the So- 

 lution retains but a fmall part of the copper loft by the plate. 

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

 precipitated fcizes, perhaps as itrongly as in the former cafe, 

 a part of the iron, or at lead its precipitation is determined 

 by the affinity of the iron for the gold, at the furface of which 

 this laft com bines. For the gilding is a combination of the 

 two metals at the furface of contact: when the firft flratum 

 is formed, the precipitation is afterwards capable of continuing 

 by the mere force of the mutual affinity which the particles 

 of gold have for each other, and which makes them to cohere. 



7. What I have juft {kited is proved in the precipitation 

 of copper by iron. When a folution of copper is decompofed 

 by a plate of iron, and the copper attached to the plate is 

 feparated, it is perceived, by the brown colour of the interior 

 furface, that the copper is not there pure, but contains fome 

 iron. After this firft ftratum the copper continued to preci- 

 pitate by contracting an adherence with itfelf, and afterwards 

 with thofe coats which are fucceffivelv formed : the affinity, 

 then, for the iron began the efiecl, which was continued by 

 that of the copper for itfelf. 



8. Phofphorus precipitates feveral metallic folutions, as has 

 been explained bv Sage and Bouillon [Journal de Phjjique 

 1781). Though it has a ftrong affinity with oxygen, yet 

 what has juft been explained relative to precipitation by the 

 metals, ought to be applied to its action. Pelletier has proved 

 that phofphorus ha3 the property of combining with metals, 

 fo that a part of that which is put in action is capable of com- 

 bining with oxvgen, while the other acls equally upon the 

 metal. There are fome metallic folutions which are not af- 

 fected by phofphorus ; in others the metal is precipitated in 

 oxide, which doubtlefs retains part of the acid of the folu- 

 tion, or of the phofphoric acid which is formed; in others 

 the metal is at length reduced. Gold, filver, copper, and 

 mercury, are among thofe which refume the metallic ftate. 

 On confidermg the obfervations which have been made on 

 this precipitation, it appears that copper and filver are preci- 

 pitated by combining with a fmall portion of phofphorus;. 

 thus, in precipitating twelve grains of filver, three grains of 

 phofphorus are coniumed ; but onlv shout three grains ofphof- 

 phoric acid, in a gelatinous ftate, are obtained: now, only 

 one grain of phofphorus is neceilarv to produce this quantity 

 of acid ; coniequently, more than two grains muft have 

 combined with the filver. A part only of the mercury, thus 

 treated, reforhes the metallic ftate ; the reft preferves the ftate 

 of an oxide, and combines with the phofphoric acid. Here 



the 



