20 The Extraction of Gold. 
may be advisable to state the reasons which induced a differ- 
ent line of experiment. 
The first attempt to secure this gold was by means of 
fine grinding and amalgamating with the following results. 
By re-crushing the tailings with mercury in a Chihan mill 
about twenty-five per cent. of the gold was obtained with 
very careful working. Im the arastra, with grooves’ in the 
bottom of the basin for mercury, the return was increased to 
thirty-three per cent. of the assayed contents. Several other 
plans based on the same principle of regrinding the sand in 
mercury gave similar results; and even with careful hand | 
amaloamation, when the material operated on was clean 
pyrites, we could not make much improvement on the above 
return. Dr. Percy, of the English Sehool of Mines, to whom 
this matter had been referred, states that from pyrites con- 
taining twenty-five and three-quarter ozs. of gold per ton, 
he only obtained eight and three-quarter ozs. when ground 
with mercury, but by roasting the pyrites before eperatine 
he obtained nearly twenty-five ozs. per ton. A sample of rich 
pyritous quartz tried by me a few months ago was ground 
to a fine powder and then rubbed up with mercury and hot 
water to extract all the free gold. The sulphides were then 
separated by hand washing, reread and amaloamated, when 
they yielded at the rate of one hundred and forty ozs. of gold 
per ton. In the two last-mentioned cases the process was 
conducted with a care and perfection such as could not be 
carried out on a large scale, and they are only quoted as a 
sample of numerous trials all tending to the same result, and 
indicating that decomposition of the pyrites was a necessary 
preliminary to any plan for extracting a fair proportion of — 
the gold. But even were it otherwise, the action of the 
sulphur and arsenic on the mercury would prevent the sul- 
phides from being treated in their raw state. 
The advantage gained by decomposing the sulphides has 
long been known and acted on in working the pyritous gold 
veins of North and South America, where the sulphides were 
piled in heaps, and subjected to natural decomposition in the 
open air for twelve months, and then re-treated to extract 
the gold that had been liberated, this process being repeated 
several times until the gold was exhausted. But it must be 
evident that a plan requiring the lapse of several years be- 
fore the gold in the waste can be rendered available, is not 
suited to the conditions under which gold miming is carried 
on in this colony, at the same time the expense “and loss of 
