Marcu 11, 1897 | 
WATLURE 
449 
diminishes slowly as the concentration is increased, and rapidly 
if it is decreased. 
These results are in perfect accord with the experience gained 
in practice ona large scale. Before Maclaurin’s papers were pub- 
lished, the favourite stock solution in South Africa had for some 
time been one containing from 0°25 to 0°30 per cent. of cyanide, 
although weaker solutions are also used with excellent effect. 
Moreover, the difficulties introduced by a lack of free oxygen in 
the ore have long been severely felt. In particular, when con- 
centrates containing much pyrites are treated, the absorption of 
oxygen by the sulphides is so rapid that the dissolution of gold 
is soon checked and becomes extremely slow. Thus, while gold- 
leaf floating on cyanide solutions is dissolved in a few minutes, 
and, if submerged, in a few hours, the films of gold in pyrites, 
which are probably similar in thickness to gold-leaf, often take 
two or three weeks in going into solution, 
This is so far from satisfactory that many efforts have been 
made to increase the speed of action of cyanide in some way. 
An artificial supply of oxygen, or air forced through the charge 
of ore and solution, was found to shorten the time required but 
to increase the waste of cyanide, and similar results follow from 
the use of various oxidising agents, such as manganese dioxide, 
hydrogen peroxide, and bleaching powder. 
Greater interest attaches to the proposal made by Sulman and | 
Teed to add bromide of cyanogen to ordinary cyanide solutions | 
(Trans. of the Zyst. of Mining and Metallurgy, vol. iii. (1895), 
p- 202). They put forward the equation 
CyBr + 3KCy + 2Au = 2KAuCy, + KBr 
as expressing the action which takes place, but no direct proof 
has yet been afforded of the validity of this equation. These 
experimenters are, however, convinced that oxygen plays no 
part in the action, and, consequently, that except for the fact that 
the edges only of the films of gold in pyrites are presented for 
attack, solution of the gold in concentrates is as rapid as that in 
quartzose ores. This view is borne out by a number of trials 
on half-ton lots of ore, although no results of actual working 
have yet been published. Even when there is full access of air, 
however, asin the case of gold-leaf floating on the solution, the 
addition of cyanogen bromide greatly increases the rate of action 
of potassium cyanide, and if air and bromine are together passed 
through a solution of cyanide, the rate is increased about 100 
times. . 
Slowness of action in dissolving gold is of more importance 
than may at first sight appear, for it must be remembered that 
alkaline cyanides attack many of the constituents of gold ores 
at varying rates, and, therefore, that the longer the solution is 
left in contact with the ore, the greater will be the decomposi- | 
tion of the cyanide, and, consequently, the greater the total cost 
of the process. Decomposing pyrites (especially if sulphide of 
copper is present) are, when not in perfect contact with 
gold, particularly active in destroying cyanide, and, in 
order to partially prevent their effect, it is customary to 
neutralise ores which have been rendered acid by the formation 
of sulphates by ‘* weathering.” The neutralisation is effected 
by the addition of a solution of caustic soda, or, more usually, of 
lime to the ore before it is treated with cyanide. The destruc- 
tion of cyanide, however, still goes on to a limited extent in 
such cases, and treatment is rendered practicable only by the 
preferential or more rapid action of very dilute solutions of 
cyanide on gold as compared with their action on the sulphides. 
The relative rates of action of cyanogen bromide on gold, and 
on the various sulphides and oxides met with in ores, remain un- 
determined, or at any rate unpublished, and until more light is 
thrown on these, either by laboratory experiments or by practical 
work on a large scale, it is impossible to judge what may be the 
future of the process. From some experiments, already made, it 
would appear that cyanogen bromide suffers considerable de- 
composition when placed in contact with some of the minerals 
met with in gold ores, and so it may happen that in many cases 
the haloid compound will be destroyed before it has time to get 
fairly to work in dissolving the gold. 
The gold is recovered from solution either by its precipitation 
and replacement in solution by a metal positive to it in 
cyanide solutions, or by electro-deposition. The only metal 
largely used in practice is zinc, the action being one of direct 
replacement, expressed by the equation 
2KAuCy, + Zn =2Au+ K.ZnCy,. 
It has been found necessary to use the zinc ina fine state of 
division, and the filaments, prepared by turning zinc in a lathe, 
NO. 1428 VOL. 55] 
are certainly more efficient than other forms. The shavings are 
no more than O°l m.m. in thickness, and 0°5 m.m. in width. 
When packed in spongy form they weigh about six or seven lbs. 
per cubic foot and can be ignited by a lucifer match, burning 
readily to zinc oxide. They must be freshly turned, as in practice 
the cyanide solutions are too dilute to clean dirty surfaces by 
dissolving hydrates or carbonates of zinc. 
The action of the zinc is undoubtedly aided by the presence 
of lead, which exists as an impurity in commercial zinc to the 
extent of about 1 per cent., and by the iron gratings on which the 
filaments rest. Galvanic couples are thus formed, which assist 
in starting the action. Nevertheless, precipitation of the gold is 
at first slow, especially in very dilute solutions, and it is only 
after some gold has been thrown down, and the gold-zinc 
couple formed, that the action becomes fairly vigorous. When 
the amount of gold in solution has fallen to about 00003 per 
cent., or from 14 to 2 dwts. per ton of liquid, the action again 
becomes slow, and this amount is left unprecipitated in practice, 
but, as the solutions are used again on fresh charges of ore, no 
loss of gold occurs. 
The black slimy deposit of gold, or alloy of gold and zinc 
thus formed, is washed and sieved off from the undecomposed 
zinc as far as possible, and is dried, roasted, and melted down 
with borax, carbonate of soda, and other fluxes, with or 
without a previous treatment with dilute sulphuric acid. The 
bullion thus obtained is very base, containing about 700 of gold 
per 1000, and variable quantities of zinc, lead, copper, and 
other metals. It is subject to the disadvantage that assay 
pieces, taken in the ordinary way, frequently differ in composi- 
tion from the ingot taken as a whole. 
| The recovery of the gold from cyanide solutions by electro- 
deposition is the basis of the Siemens-Halske process. In 
this process the kathodes are of iron, and the anodes of lead 
foil. A very large surface is given to the electrodes, 12,000 
square feet of surface of lead being exposed in the treatment of 
70 tons of solution per day at the Worcester Mine in the 
Transvaal. At stated intervals, the lead anodes, containing 
from 2 to 12 per cent. of gold, are removed, melted down, and 
cupelled. The bullion produced is very fine, but the cost of 
precipitation appears to be greater than that by the zinc process, 
the main items being the lead and iron consumed. The 
current needed is only about 0°06 ampere per square foot, the 
power required being about 5 h.p. in the treatment of 70 tons 
of solution per day. The process makes but slow progress, 
| only a small proportion of the gold produced by cyanide on 
the Rand being obtained in this way. K. RoseE. 
AGRICULTURAL TEACHING AT OXFORD. 
THE present Professor of Rural Economy at Oxford has made 
use of the opportunity, afforded by the occurrence of the 
centenary of the foundation of his professorship, to discuss the 
general question of agricultural teaching in our Universities. 
After an interesting description of the life and work of Sibthorp, 
formerly Professor of Botany at Oxford, who literally gave up 
his life for the study of natural history on the shores of Greece, 
Prof, Warington proceeds, in the first place, to consider the 
important developments, both in the subject and in the means 
of teaching, which have taken place during the past century. 
He says :-— 
‘<The point on which I want to fix attention is the wholly 
different position in which agriculture stands at the present day 
from that which it held a hundred years ago. A hundred years 
ago agriculture was an art, having few points of contact with 
natural science. At the present time, both the materials and 
the operations of agriculture have been so far examined and 
elucidated by patient scientific investigation, that we may now 
fearlessly give the title of ‘ Agricultural Science’ to the edifice 
of true theory which has been constructed. We need not 
shrink from making this claim because the theoretical edifice 
is still incomplete, for this incompleteness of theory is the 
normal condition of the natural sciences; what we assert is, 
that the whole field of agriculture is now occupied by the 
1 “ Agricultural Science : its place in a University Education.” A lecture 
delivered before the University of Oxford, on November 5, 1896, on the 
| oceasion of the centenary of the foundation by Dr. John Sibthorp of the 
chair of Rural Economy, by Robert Warington, M.A., F.R.S., Sibthorpian 
Professor of Rural Economy. (London: Henry Frowde, 1896.) 
