HISTORY OF GOLD, 535 



If the reader would be still more fully convinced of the real Further prooft 

 difference, that subsists between the auriferous precipitates and <^» y^'^l "i"«i" 

 the purple powder of Cassius, it is sufficient to remind him, 

 that, though the muriate of iron acts upon the muriate of gold 

 in the same manner as that of tin, the oxide of iron, which 

 occasions the precipitation, is by no means attracted by the- 

 gold like the oxide of tin, though its solubihty is as strikingly 

 diminished. If, then, the oxide ot iron, raised to its maximum, 

 do not unite with the gold when they come together under the 

 circumstances in which that of tin infalUbly does, certainly 

 nothnig but the power of affinity can account for the differ- 

 ence. 



Lastly, the purple prepared with tin fixes in silk, and dves Pu'^P^^of^'^** 

 •^ ^ / ^ ^ . , •; sius dyes silk. 



It of a violet colour; which is certainly not the effect of a 



powder of gold incorporated in the pores of its filaments. 



In the formation of the purple powder, the excess of the Effect of the 

 acids has an office veiy different from what might be supposed; ^^L 

 the acids do not take from it any oxide of tin, as it was natural 

 to expect, but they cause, or communicate to the purple pow- 

 der, a kind of dernisolution, which retards its subsidence, and 

 which is on this account veiy inconvenient, particularly when 

 we are in haste to obtain it. This action of the acids may 

 easily be perceived, if we agitate in marine acid a precipitate 

 recently washed : on doing this, you would say, a solution 

 takes place, and still more if a little heat be employed ; but 

 this solution, which imposes on the eye by a kind of transpa- 

 rency, dees not stand the test of the filter. 



The preci['itation of the purple powder may be accelerated Potash hastens 

 by adding potash occasionally to the liquor. If, at the expi- ^j^^^^^^'^'^''^' *' 

 ration of a few minutes, its surface does not become clear, a 

 little more may be added ; and the operator will have the 

 pleasure of seeing the purple powder collecting in clouds and 

 visibly subsiding. In this precipitation, however, an excess _but an excess 

 is to be apprehended ; but this is easily avoided when we are ^^^^ beavoii- 

 aware of it. The fact is, if m-ore potash than is necessary to 

 neutralize the surplus of acid be employed, it will act on the 

 iincombined muriate of tin held in solution; and thus a portion 

 of oxide will be added to the purple powder, which should 



I jnake no part of it. 



* ■ Bfect 



