230 



would pi^ecipitate along with the gold by a simple chemical substitution. On the 

 other hand, silver woiald readily be absorbed by inferior sulphides, and also by 

 galena. 



All tliis agrees very well with what we find on examination of these sul- 

 phides ; indeed, the facts thus stated seem to explain at once the reason of the 

 almost constant association of gold and silver with certain of our metallic 

 sulphides, and their absence or comparative scai-city in others. 



Whether such is the true explanation of this or not, the knowledge that 

 metallic sulphides and arsenides are capable of reducing several metals from their 

 solutions, should, I think, be taken into consideration when we attempt to 

 explain the origin of these metals in the forms they have taken, and in the 

 rocks or veins they have selected. 



As yet, most of the theories broached for the explanation of the occurrence 

 of such deposits in these matrices, are based upon the reducing power of 

 organic matter ; when the fact is, that most or all of these suljihides are much 

 superior in this respect to such matters, being far more rapid in their effects, 

 and capable of reducing, weight for weight, more metal ; a single grain of iron 

 pyrites being sufficient to reduce S^ grains of gold. 



Besides, organic matter could scarcely exist in such quantity among our 

 older and more altered rocks, or be carried there in such quantity as to effect 

 the reduction of gold in such qiiantity and in such confined spaces as it is 

 occasionally discovered in. 



While therefore allowing that organic matter may have had a share in the 

 reduction of this and other metals, I cannot but think that by far the greater 

 portion of these deposits — especially those deeper seated ones at a distance from 

 carboniferous strata — have been due to the deoxydizing effects of pyritous 

 minerals. 



Of course both theories recjuire, as a common ground, that the metal was 

 first in the form of a salt, soluble in water — a condition which is generally 

 conceded, and which I have therefore all along taken as actually obtaining. 



I cannot take leave of this subject without adverting to, and commenting 

 upon, some of the singvilar chemical reactions which these researches have 

 opened up. 



In the first place we learn that aqueous solutions of the oxy-salts of gold — 

 among which I include the chlorides, for reasons above stated — possess oxydizing 

 power to an extent not hitherto contemplated for them, great as this may have 

 been acknowledged. I think it is a little superior to that of chromic acid — 

 one of the most powerful oxydizing agents we have — since it attacks cubical 

 pyrites pretty readily in the cold, while chromic acid scarcely affects it. 



Further, these gold salts not only oxydize the metal of these sulphides, but 

 their sulphur too ; though I find from actual experiment that common sulphur 

 (crystallized), or recently precipitated sulphur, does not appear to be the least 



