ir ow 
Feb. 1, 1883] 
NATURE 
327 
corrosion and to deposit the whole of the silver from a solution 
of argento potassic cyanide of given composition and containing 
free cyanide are also shown. ‘The influence of varying the pro- 
portions both of argento potassic cyanide, and free cyanide of 
potassium, upon the transfer resistance! of the solution, and 
thereby upon the balance point, are also investigated and the 
results described. 
A number of results and conclusions were arrived at, some of 
which are as follows :—variation either of the number of battery 
elements, the proportion of water, of free potassic cyanide, or 
of argento potassic cyanide, destroys the balance. The effect of 
altering the proportion of water is opposite with strong solutions 
to what it is with weak ones. ‘The electric current at the point 
of balance appears to be entirely conveyed by the free potassic 
cyanide, and does not divide itself between the two salts until 
the liquid contains a certain proportion of argentic salt. In 
strong solutions of potassic cyanide, decreasing the number of 
battery cells, necessitates more cyanide of silver to restore the 
balance. The alteration of the point of balance by alteration 
of proportion of free potassic cyanide cannot be much accounted 
for by alteration of corrosive power of the liquid. A current 
from ten Smz2e’s elements is about sufficiently strong to prevent 
all corrosion of silver at 60° F. in a solution of cyanide of 
potassium containing a mere trace of argento potassic cyanide. 
The addition of nitrate, chloride, iodide, or sulphate of potassium 
to the cyanide solution has but little effect upon the balance 
point. Variation of strenith and of ‘‘density” of current 
affect greatly the point of balance. Greater ‘‘ density” irre- 
spective of strength of current usually increases the amount of 
silver deposited. Diffcrence of electro-motive force of current 
had no conspicuous effect in altering the balance point. Rise of 
temperature of the liquid acts in two opposite ways, it increases 
the corrosive action, and by diminishing conduction-resistance 
it increases the current, and as the latter effect is usually a little 
stronger than the former one, rise of temperature alters slightly 
the point of balance, and enables the current to produce a sparing 
deposit of silver. ‘The ordinary chemical corrosion of silver in 
a solution of potassic cyanide without an electric current is 
increased slightly by partial immersion (through capillary 
corrosion), and greatly by rise of temperature ; it is also slightly 
greater in a weak solution than ii a strong one, with solutions of 
a certain range of strength ; and it is distinctly increased by 
contact with platinum. In consequence of the latter circum- 
stance, a platinum cathode requires a somewhat stronger current 
than a silver one to enable the point of balance to be attained. 
In a mixed solution of potassic and argento potassic cyanides, 
even the smallest proportion of the former salt conveys a portion 
of the current, and if the cathode is large or the current is 
sufficiently weak, the whole of it is conveyed by thai salt, how- 
ever much of the double salt is present, an error is thereby 
introduced when deposition of silver in such a liquid is used as a 
measure of current. But with a large amount of the double 
salt, a small amount of potas:ic cyanide, anda current sufficiently 
strong, the proportionate amount of error is small. During the 
act of deposition the cathode surface is not at all corroded, and 
any deficiency in the weight of deposit is not due to corrosion, 
but to a portion of the current being conveyed by other ingredients 
of the liquid than the argentic salt. A current which produces 
deposition of silver, prevents all corrosion of a silver cathode in 
the same liquid. The addition of free potassic cyanide to a 
solution of the double cyanide alters both the resistance and the 
balance point. The quantity of current diverted from the 
argentic salt in solution 1s directly proportional to the amount of 
free potas-ic cyanide present, but not always in the same ratio, 
The presence of a large proportion of free cyanide, together 
with the employment of a feeble current conduce to the passage 
of a large amount of current through the liquid without 
depositing silver ; and a current of ‘001057 Ampere (which 
would deposit *132 grain of silver in two hours) was hardly 
strong enough to prevent all corrosion or to deposit any silver 
from a solution composed of 37°5 grains of argento potassic 
cyanide and 112°5 grains of free cyanide of potassium in three 
ounces of water. Whilst also a current if sufficiently weak, 
may traverse a solution of potassic cyanide containing double 
cyanide, without any of the current decomposing the latter, it 
cannot traverse a solution of double salt containing free potassic 
cyanide without some of it traversing the cyanide of potassium, 
_ * By transfer resistance is meant the resistance to transfer of the current 
into the cathode. 
With a very dense current also, a portion of it enters the cathode 
without depositing silver, and evolves gas. 
It requires a much stronger current to balance the corrosion in 
a hot solution of the two cyanides than in a cold one, and in an 
instance given, a rise of temperature from 60 to 120° F, was 
attended by the passage of 21 per cent increase of current 
without deposition of silver, Addition of free potassic cyanide 
to a weak solution of the double salt at the balance point, 
first decreases and then increases the current by altering the 
transfer resistance, probably at the cathode. An amount of 
current equal to ‘14857 Ampere, entering a surface of ths, 
of a square inch, was found to be sufficiently strong to 
deposit nearly the whole of the silver from a solution at 60° F, 
composed of 701! grains of free potassic cyanide, ‘0297 grain 
of double cyanide, and three ounces of water, the liquid retain- 
ing dissolved a little less than that amount of silver at its balance 
point under those conditions. The strength of current at the 
balance point in a weak solution of potassic cyanide, varies 
inversely as the amount of silver salt added, and at about eight 
times the rate. A c rtain strength of current must enter a given 
surface of silver in a given liquid under stated conditions in 
order to prevent all corrosion and produce deposition. The 
addition of the double cyanide reduces the amount of current con- 
veyed by the free pcotassic cyanide into the cathode at the balance 
point. Successive additions of double salt to a solution of 
potassic cyanide xot at the balance point, first decreases and 
then increases the current by altering the transfer resistance ; 
it alters the relation of the molecules of potassic cyanide to the 
cathode so as to diminish their power of transmitting current into 
that surface without depositing silver. The greater the proportion 
of double salt present, the greater the tendency to the deposition 
of silver. Addition of potassic cyanide to a weak solution of 
the double salt of at the balance point, first decreases and then 
increases the current by altering the transfer resistance at the 
cathode ; in this respect it behaves like addition of the double 
salt to a weak solution of potassic cyanide. With cathodes of 
platinum, a solution of potassic cyanide offered less resistance to 
the current (sot at the balance fotnt) than one of the double 
cyanide, but with silver cathodes the reverse effect occurred. 
The balance point is a case of equalization of molecular 
influences, including ordinary chemical corrosion, density of 
current, nature of cathode, temperature, proportions of water, 
argento potassic cyanide, free potassic cyanide, and the soluble 
salts present as impurities, either of which by being disturbed, 
alters all the others, All these influences also have separate 
numerical values. A rise of temperature of 60° F. requires 
an increase of ‘000976 Ampere to restore the equipoise. 
The experiments illustrate the dynamics of electro silver 
plating ; and the method employed in the research is appli- 
cable to the detection and measurement of molecular influ- 
ence in electrolytes. In consequence of the alteration of 
any one of the conditions having the effect of altering all the 
remainder, all the above conclusions are limited in their applica- 
tion and are only correct under the conditions given in the paper. 
The fundamental explanation underlying these conclusions is, 
that the phenomena are essentially molecular ; and that the mere 
presence and admixture of the double cyanide alters the mole- 
cular arrangement of the free cyanide not at the balance point, 
in such a way as to enable the latter to transmit a greater 
quantity of current into a cathode of given size, notwithstanding 
its being more diluted by the other salt. 
The phenomena of the ‘‘balance point” constitute an 
interesting example of molecular equilibrium, in which the 
balance point may be compared toa ball suspended by an 
elastic cord, and having attached to it, a number of other 
similar cords, each drawing it ina different direction, and all of 
them being kept in a state of tension. In sucha case an alter- 
ation of the degree of strain of any one of the cords, changes 
that of all others, and alters the position of the ball. 
The research has a practical bearing both upon the measurement 
of electric currents by means of deposition of silver froma cyanide 
solution, and upon the technical process of electro plating. In 
the former it shows how a large proportion, or even the whole of 
a current may pass without being measured, and how the error 
may be reduced to the smallest amount ; and in the latter, how 
a similar waste of current may occur, and how to prevent it. 
It is manifest from the foregoing research, that the electrolytic 
balance of chemical corrosion of cathodes in other depositing 
solutions, such as those of gold, copper, nickel, etc., might form 
an extensive subject of experimental investigation. 
