380 Transactions. — Chemistry. 



such matters disseminated, or even topping tliem in actual contact with the 

 flow of water. 



It follows then from what has been previously affirmed, that there will be 

 a reduction of gold by these nodules, and that the metal thus reduced will be 

 firmly attached to them, at first in minute spangles isolated from each other, 

 but afterwards accumulating and connecting in a gradual manner at that 

 point of the pyritous mass most subject to the current, until a continuous 

 film of some size appears ; this being formed, the pyrites and gold is to a 

 certain extent polarized, the film or irregular but connected mass of gold 

 forming the negative, and the pyrites the positive end of a voltaic pair ; and 

 so according as the polarization is advanced to completion, the further deposi- 

 tion of gold is changed in its manner from an indiscriminate to an orderly 

 and selective deposition concentrated upon the negative or gold plate. 



The deposition of gold being thus controlled, its loss by dispersion or from 

 the crumbling away of the sustaining pyrites is nearly or quite prevented — a 

 conservative effect, which we could scarcely expect to obtain if organic matter 

 were the reducing agent. 



Meanwhile there is a gradual wasting away of the pyrites or positive pole, 

 its sulphur being oxidized to sulphuric acid, and its iron to sesqui-oxide of 

 iron or hematite, a substance very generally associated with gold nuggets. 

 According to the original size of the pyritous mass, the protection it receives 

 from the action of oxidizing substances other than gold, the strength of the 

 gold solution, length of exposure to it, and rate of supply (velocity of stream), 

 will be the size of the gold nugget. 



As to the size of a pyritous mass necessary to produce in this manner a 

 large nugget, it is by no means considerable. A mass of common pyrites 

 (bi-sulphide of iron) weighing only about twelve pounds being competent for 

 the construction of the famous " Welcome nugget," an Australian find, having 

 weight equal to 152 lbs. avoirdupois. 



Such masses of pyrites are by no means uncommon in our drifts or the 

 beds of our mountain streams. The general velocity of the current flowing 

 over such pyritous matters would in all probability be such as would prevent 

 the development of any crystalline form in the gold thus deposited, as we 

 know very well that for such development motion is unfavourable. The form 

 most likely to be assumed by these deposits then would be the mammillary, 

 precisely that in which our nuggets as a rule occur. 



Upon this mode of accounting for the presence of large nuggets in our 

 drifts, their occasional great superiority in point of size to any auriferous mass 

 as yet found in our reefs, and their superior fineness to such reef gold, admits 

 of easy explanation. 



Firstly, as regards their comparative size, if we only admit that reef gold 



