288 STATE AND PROGRESS OF 
of silver, or 74 % of the value of the original ore, and was almost 
entirely composed of argentiferous zinc blende. With any of 
these concentrating machines the heavy pyritous minerals can be 
concentrated from the ores, leaving the tailings almost absolutely 
clean, but since every ore requires special adjustments of the 
concentrators to achieve the best results, it is only by actual trials, 
which may take a few days, that the most perfect adjustment 
can be arrived at. Where vanners are employed it is necessary 
to crush with as little water as possible, and consequently the 
tables have to be set on a steep grade. If this be not attended 
to there is too much water for the vanners to work satisfactorily. 
It is the subsequent treatment of the concentrated sulphides 
that has, perhaps, received the greatest amount of attention of 
late, as the colonies have but recently awakened to the fact that 
on this treatment the ultimate success of the gold-mining 
industry depends. In every case the pyrites has to be roasted 
in the first instance, with the exception of the so-called cyanide 
process, in which the finely-divided sulphides are digested with 
potassic cyanide, which is stated to dissolve the silver and gold, 
and these are subsequently precipitated by zinc, the cyanide 
being recovered. We are not aware that this process has been 
tried on a practical scale in the colonies, but the results of 
laboratory tests by several observers has disclosed the fact that 
the results are very various, and while sometimes nearly 90 /% of 
the silver and gold are obtained, in other cases not more than 
half that proportion is saved. It would appear, therefore, that 
there are some disturbing agencies which are not yet thoroughly 
understood, and consequently that the process requires further 
investigation before it can be considered a practical success. 
Roasting is the first requisite in all other processes, the object 
being to oxidise the sulphides and liberate the gold in a free state, 
and this is done at several of the mines. There are certain 
difficulties attached to roasting ores, some relating to the question 
of expense and others to the complete extraction of the gold and 
silver if it is present. 
The simplest form of furnace, and the one which is usually 
employed here, is the reverberatory, the floor of which is made 
very long, and the ore being fed through a hopper at the end 
farthest from the bridge, is gradually raked down until it reaches 
the hottest part near the flame, and from this point it is scraped 
through a hole in the floor into cars, which convey it to the cooling 
chamber. Roasting in this furnace can be done as perfectly as in 
any other, but the expense of handling, and the hard work it 
entails on the men, is a decided disadvantage, and a good deal of 
attention has been directed elsewhere to the construction of 
furnaces which obviate these difficulties. We may mention 
Bruckner’s revolving cylinder, White’s, White-Howell and 
Howell’s improved revolving furnaces, which have been largely 
