120 Pelletier on Gold. 



great excess, the precipitate will form, but then in a great 

 measure disappear, and the portion which remains undissolved, 

 instead of being yellow and gelatinous, is black and pulver- 

 rulent. In this state it contains no water, the potash having 

 completely separated it. It is principally this difference in the 

 colour of the oxide, obtained in circumstances opposed to each 

 other, which induced M. Oberkamp to consider the black oxide 

 as a pure oxide, and the yellow oxide as a sub-salt. The ex- 

 periments of M. Pelletier prove this difference to be un- 

 founded. 



It is remarkable that the solution of gold, thus treated with 

 excess of potash, entirely loses the colour of the salt of gold, 

 and becomes colourless, so ^s to prevent the suspicion of the 

 presence of any of this metal. The gold, however, is by no 

 means perfectly separated ; what, therefore, has become of that 

 left behind ? or how is it retained in solution ? It appears dif- 

 ficult to give an explanation of these effects. It is certain that 

 the yellow colour of the solution is restored by the addition 

 even of weak OjCid. M. Pelletier does not think with M. Vau- 

 quelin, that a triple colourless salt is formed, but in conse- 

 quence of experiments, described in his Memoirey accounts for 

 the appearances otherwise. It seems that the excess of alcali 

 on decomposing the chloride of gold perfectly re-dissolves the 

 oxide of gold formed, and that a binary colourless compound 

 results, in which the oxide acts as an acid to the alcali, so 

 that an aurate of potash remains in solution, mixed with chlo- 

 ride of potasium. It is evident from this fact, that the addi- 

 tion of an acid will decompose the aurate, by taking the alca- 

 line base ; and that the oxide of gold, set at liberty, will re- 

 act on the chloride of potassium, and re-produce chloride of 

 gold, the colour of which will then become visible. In these 

 circumstances a portion of alcali is set at liberty, and the fluid 

 becomes alcaline independent of the alcalinity of the aurate 

 remaining undecomposed. The following remarkable experi- 

 ment may be quoted in support of this statement : — If oxide of 

 gold be boiled in a solution of chloride of sodium, the fluid be- 

 comes yellow and alcaline* 



