lEON PTBITES. 39 



were greatly assisted by the knowledge gained of the physical 

 condition of the gold in pyrites. Consequently, after our earlier 

 experiments, already mentioned, we carefully avoided a current 

 of water and ground with mercury without an overflow until it 

 was considered the gold had become amalgamated. By cautiously 

 varying the experiments, and ascertaining the proportion of gold 

 obtained in each case, we were led up to the present method of 

 working. From the results of a great number of experiments, 

 it was shown that the quantity of water used in the grinding 

 process was a matter of considerable importance, the success of 

 the operation to a great extent depending upon the sand being 

 in a damp condition only ; by this means the mercury becomes 

 thoroughly diffused, and every grain of sand has a jDarticle of 

 mercury in contact with it, consequently there is afforded abun- 

 dant opportunity for the gold to amalgamate. On the other hand, 

 if sufB.cient water has been used to convert the mass into a 

 semi-fluid state, the mercury remained at the bottom of the mill, 

 the surface only being in contact with the sand, consequently 

 the opportunity for amalgamation was considerably lessened, and 

 the quantity of gold extracted very much less than when working 

 damp sand only. As the result of these experiments, the fol- 

 lowing process has been adopted : — The sand, after being roasted, 

 is ground — only moderately damp — with an equal weight of 

 mercury for three-quarters of an hour, under the rollers of a 

 heavy Chilian mill ; water is then allowed to flow into the basin, 

 the mill still revolving, until nearly all the finely ground sand 

 has been carried off in the overflow to the concentrator ; the mill 

 is then stopped, the water drained off from the unground 

 sand and mercury, again started, and recharged with fresh sand, 

 and so on until it is necessary to clear out the amalgam — gen- 

 erally once a week, depending upon the richness of the mineral 

 treated. The finely ground sand passed into the concentrator is 

 kept slowly stirred for a quarter of an hour, to keep the sand in 

 suspension in the water, and allow the mercury and any amalgam 

 which might have been carried over from the mill to gravitate 

 through it ; the water with its sand is then slowly run through 

 a smaller concentrator, to retain any valuable particles which 

 might have escaped the first, and is then considered sufficiently 

 impoverished to be allowed to run away. The quantity of roasted 

 mineral the Company treat by this process, when working, 

 averages 18 tons per week — the duty of two mills worked in 

 eight-hour shifts, one man attending them, alternately charging 

 and discharging. 



The average proportion of gold extracted during last year, 

 from 294 tons of pyrites, amounted to 95 "19 per cent, of the 

 assays, the sand averaging 4| oz. of gold per ton. Some of the 

 parcels returned as high as 98 per cent, during that period. The 



