35A MISTORY OF G0Li5. 



metal, the affinities of which are so limited ; with an oxide, the 

 affinities of which are no less injured hy e.xtrenae oxidation, 

 canno-t fail to appear singular ; particularly as, I believe, there 

 does not exist any that can be compared with it. It may be 

 objected, that the combination of the oxide of gold with, 

 ammonia will not appear, perhaps, less anomalous, if it be x 

 closely investigated : for what can be more opposite to the 

 general principles of chemistry, than to see an oxide having a 

 stronger affinity to ammonia than to the most powerful acids } 

 or, than to see in fulminating gold an oxido-ammoniacal 

 combination, which no acid, or even alkali, can overcome ? 



Mercury can- JMercury, shaken in a phiai with fresh purple of Cassius, does 



not take gold ^^^ ^^y^^ f,.yj^ if ^^^ y ^^j^j^ which it unites so advan- 



trom the pur- ° ' 



pie of Cassius. tageously on all occasions where there is no combination. If, 



however, actually metallic gold in the i)urp!e powder do not 

 yield to the action of mercury, some other affinity must cer- 

 tainly interpose. 

 Differs from It has been thought too, that there is no essential difference 



bv sulphate of between gold precipitated by tin, and gold precipitated by 

 Moa; sulphate of iron ; and even that the latter only required to be 



mixed or diluted with a while oxide of any kind, to become 

 the purple powder ot Casbius. Those notions, 1 must say, 

 have very little foundation : tor if gold, precipitated by iron, 

 have something of a purple hue when depobited, it has this tint 

 only in one pt.int ot view exclusively; while that of Cassjus is 

 constantly purple, in whatever position we look at it. 



— and from all All gold, reduced to the metallic state by any other com- 



othtr precipi- , , , , , . , , 



tates by cum- bustible substance than tin, } lelds a precipitate, every atom 



bust ible sub- of which refracts the light, so as to make it appear blue when 

 the vessel containing it is placed between the light and eye. 

 Gold, precipitated by sulphate of iron, by phosj)horated hidro- 

 gen, by sulphureous acid, 6ic., has this appearance. If we 

 stand between the vessel and the light, we still perceive no 

 purple ; but we distinguish the reflection peculiar to each au- 

 riferous molecule, in which we clearly recognize particles of 

 metal. But gold, precipitated by tin, on the contrary, is of a 

 deep crimson, is purple, is a velvety powder, which yields no 

 metallic reflection, under whatever angle it is examined, and 

 the tint of which diticrs from that of its solution in ammonia 

 onl} by being more intense. 



If 



