Chio, Turkey.
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ON THE CHANGE OF VOLUME WHICH ACCOMPANIES THE GALVANIC DEPOSITION OF A
METAL.
By M. E. BOUTY.
In previous notes I have established, first, that the galvanic
depositions experience a change of volume, from which there results a
pressure exercised on the mould which receives them; second, that the
Peltier phenomenon is produced at the surface of contact of an electrode
and of an electrolyte. Fresh observations have caused me to believe that
the two phenomena are connected, and that the first is a consequence
of the second. The Peltier effect can clearly be proved when the
electrolysis is not interfered with by energetic secondary actions, and
particularly with the sulphate and nitrate of copper, the sulphate and
chloride of zinc, and the sulphate and chloride of cadmium. For any one
of these salts it is possible to determine a value, I, of the intensity
of the current which produces the metallic deposit such that, for all
the higher intensities the electrode becomes heated, and such that it
becomes cold for less intensities. I will designate this intensity, I,
under the name of _neutral point of temperatures_.
The new fact which I have observed is, that in the electrolysis of the
same salts it is always possible to lower the intensity of the current
below a limit, I', such that the compression produced by the deposit
changes its direction, that is to say, instead of contracting the
metal dilates in solidifying. This change, although unquestionable,
is sufficiently difficult to produce with sulphate of copper. It is
necessary to employ as a negative electrode a thermometer sensitive
to 1/200 of a degree, and to take most careful precautions to avoid
accidental deformations of the deposit; but the phenomenon can be
observed very easily with nitrate of copper, the sulphate of zinc,
and the chloride of cadmium. There is, therefore, a _neutral point
of compression_ in the same cases where there is a neutral point of
temperatures. With the salts of iron, nickel, etc., for which the
neutral point of temperatures cannot be arrived at, there is also no
neutral point of compression; and the negative electrode always becomes
heated, and the deposit obtained is always a compressing deposit.
I have determined, by the help of observations made with ten different
current strengths, the constants of the formulæ which I have explained
elsewhere, and which gives the apparent excess, y, of the thermometer
electrode compressed by the metallic deposit in terms of the time, t,
during which the metal was depositing:
A t
(1) y = -------
B + t
The constant, A, is proportional to the variation of volume of the unit
of volume of the metal. The values of A, without being exactly regular,
are sufficiently well represented within practical limits by the
formula:
(2) A = - a'i + b'i²,
of the same form as the expression E:
E = - ai + bi²,
of the heating of the thermometer electrode. Further, every cause which
affects the coefficients, a or b, also affects in the same way a' and
b': such causes being the greater or less dilution of the solution, the
nature of the salt, etc. It is, therefore, impossible not to be struck
by the direct relation of the thermic and mechanical phenomena of which
the negative electrode is the origin. The following is the explanation
which I offer: The thermometer indicates the mean temperature of the
liquid just outside it; this temperature is not necessarily that of the
metal which incloses it. The current, propagated almost exclusively by
the molecules of the decomposed salt, does not act directly to cause a
variation in the temperature of the dissolving molecules; these change
heat with the molecules of the electrolyte, which should be in general
hotter than those when a heating is noticed and colder when a cooling is
observed. Suppose it is found, in the first case, that the metal, at
the moment when it is deposited, is hotter than the liquid, and,
consequently, than the thermometer; it becomes colder immediately after
the deposit, and consequently contracts; the deposit is compressed.
The reverse is the case when the metal is colder than the liquid; the
deposit then dilates. If this hypothesis is correct, the excess, T,
of the temperature of the metal over the liquid which surrounds the
thermometer should be proportional to the contraction, A, represented
by the formula (2), and the neutral point, I', of the contraction
corresponds to the case where the temperature of the metal is precisely
equal to that of the liquid.
It might be expected, perhaps, from the foregoing, that I' = I; this
would take place if the excess of temperature of the metal, measured
by the contraction, were rigorously proportional to the heating of the
liquid, for then the two quantities would be null at the same time.
Careful experiment proves that this is not the case. The sulphate of
copper gives compressing deposits on a thermometer which is undoubtedly
cooling; chloride of zinc of a density 200 can give expanding
deposits on a thermometer which is heating. There is, therefore, no
proportionality; but it must be remarked that the temperature of the
metal which is deposited does not depend only on the quantities of heat
disengaged in an interval of molecular thickness which is infinitely
small compared with the thickness of the layer, of which the variations
of temperature are registered by the thermometer. There is nothing
surprising, therefore, that the two variations of temperature,
according exactly with one another, do not follow identically the same
laws.--_Comptes Rendus._
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