The energy produced by the phenomena of diffusion is exhibited in
lecture courses by placing a bell glass filled with hydrogen over a
porous vessel at whose base is fixed a glass tube that dips into
water. The hydrogen, in diffusing, enters the porous vessel, increases
the internal pressure, and a number of bubbles escapes from the tube.
On withdrawing the bell glass of hydrogen, the latter becomes diffused
externally, a lower pressure occurs in the porous vessel, and the
level of the water rises.
The arrangement devised by Mr. C.J. Woodward, and recently presented
to the Physical Society of London, is an adaptation of this experiment
to the production of an oscillating motion by alternations in the
internal and external diffusion of the hydrogen.
The apparatus, represented herewith, consists of a scale beam about
three feet in length that supports at one end a scale pan and weights,
and, at the other, a corked porous vessel that carries a glass tube,
c, which dips into a vessel containing either water or methylic
alcohol. Three or four gas jets, one of which is shown at E, are
arranged around the porous vessel, as close as possible, but in such a
way as not to touch it during the oscillation of the beam. These gas
jets communicate with a gasometer tilled with hydrogen, the bell of
which is so charged as to furnish a jet of sufficient strength.
Experience will indicate the best place to give the gas jets, but, in
general, it is well to locate them at near the center of the porous
vessel when the beam is horizontal.
[Illustration]
It is now easy to see how the device operates. When the hydrogen comes
in presence of the porous vessel it becomes diffused therein, and the
pressure exerted in the interior then produces an ascent. When the
bottom of the porous vessel gets above the jets, the internal
diffusion ceases and the hydrogen becomes diffused externally, the
internal pressure diminishes, and the vessel descends. The vessel then
comes opposite the jets of hydrogen and the same motion occurs again,
and soon indefinitely. The work produced by this motor, which has
purely a scientific interest, is very feeble, and much below that
assigned to it by theory. In order to obtain a maximum, it would be
necessary to completely surround the porous vessel each time with
hydrogen, and afterward remove the jets to facilitate the access of
air. All the mechanical arrangements employed for obtaining such a
result have failed, because the friction introduced by the maneuvering
parts also introduces a resistance greater than the motor can
overcome. There is therefore a waste of energy due to the continuous
flow of hydrogen; but the apparatus, for all that, constitutes none
the less an original and interesting device.--_La Nature._
* * * * *
SOME RELATIONS OF HEAT TO VOLTAIC AND THERMO-ELECTRIC ACTION OF METALS
IN ELECTROLYTES.[1]
[Footnote 1: Read before the Royal Society, Nov., 1883.]
By G. GORE, F.R.S., LL.D.
The experiments described in this paper throw considerable light upon
the real cause of the voltaic current. The results of them are
contained in twenty tables; and by comparing them with each other, and
also by means of additional experiments, the following general
conclusions and chief facts were obtained.
When metals in liquids are heated, they are more frequently rendered
positive than negative in the proportion of about 2.8 to 1.0; and
while the proportion in weak solutions was about 2.29 to 1.0, in
strong ones it was about 3.27 to 1.0, and this accords with their
thermo-electric behavior as metals alone. The thermo-electric order of
metals in liquids was, with nearly every solution, whether strong or
weak, widely different from the thermo-electric order of the same
metals alone. A conclusion previously arrived at was also confirmed,
viz., that the liquids in which the hot metal was thermo-electro-positive
in the largest proportion of cases were those containing highly
electro-positive bases, such as the alkali metals. The thermo-electric
effect of _gradually_ heating a metal in a liquid was sometimes
different from that of _suddenly_ heating it, and was occasionally
attended by a reversal of the current.
Degree of strength of liquid greatly affected the thermo-electric
order of metals. Increase of strength usually and considerably
increased the potential of metals thermo-electro-negative in liquids,
and somewhat increased that of those positive in liquids.
The electric potential of metals, thermo-electro-positive in weak
liquids, was usually about 3.87 times, and in strong ones 1.87 times,
as great as of those which were negative. The potential of the
strongest thermo-electric couple, viz., that of aluminum in weak
solution of sodic phosphate, was 0.66 volt for 100° F. difference of
temperature, or about 100 times that of a bismuth and antimony couple.
Heating one of the metals, either the positive or negative, of a
voltaic couple, usually increased their electric difference, making
most metals more positive, and some more negative; while heating the
second one also usually neutralized to a large extent the effect of
heating the first one. The electrical effect of heating a voltaic
couple is nearly wholly composed of the united effects of heating each
of the two metals separately, but is not however exactly the same,
because while in the former case the metals are dissimilar, and are
heated to the same temperature, in the latter they are similar, but
heated to different temperatures. Also, when heating a voltaic pair,
the heat is applied to two metals, both of which are previously
electro-polar by contact with each other as well as by contact with
the liquid; but when heating one junction of a metal and liquid
couple, the metal has not been previously rendered electro-polar by
contact with a different one, and is therefore in a somewhat different
state. When a voltaic combination, in which the positive metal is
thermo-negative, and the negative one is thermo-positive, is heated,
the electric potential of the couple diminishes, notwithstanding that
the internal resistance is decreased.
Magnesium in particular, also zinc and cadmium, were greatly depressed
in electromotive force in electrolytes by elevation of temperature.
Reversals of position of two metals of a voltaic couple in the tension
series by rise of temperature were chiefly due to one of the two
metals increasing in electromotive force faster than the other, and in
many cases to one metal increasing and the other decreasing in
electromotive force, but only in a few cases was it a result of
simultaneous but unequal diminution of potential of the two metals.
With eighteen different voltaic couples, by rise of temperature from
60° to 160° F., the electromotive force in twelve cases was increased,
and in six decreased, and the average proportions of increase for the
eighteen instances was 0.10 volt for the 100° F. of elevation.
A great difference in chemical composition of the liquid was attended
by a considerable change in the order of the volta-tension series, and
the differences of such order in two similar liquids, such as
solutions of hydric chloride and potassic chloride, were much greater
than those produced in either of those liquids by a difference of 100°
F. of temperature. Difference of strength of solution, like difference
of composition or of temperature, altered the order of such series
with nearly every liquid; and the amount of such alteration by an
increase of four or five times in the strength of the liquid was
rather less than that caused by a difference of 100° F. of
temperature. While also a variation of strength of liquid caused only
a moderate amount of change of order in the volta-tension series, it
produced more than three times that amount of change in the
thermo-electric tension series. The usual effect of increasing the
strength of the liquid upon the volta-electromotive force was to
considerably increase it, but its effect upon the thermo-electro-motive
force was to largely decrease it. The degree of potential of a metal
and liquid thermo-couple was not always exactly the same at the same
temperature during a rise as during a fall of temperature; this is
analogous to the variations of melting and solidifying points of
bodies under such conditions, and also to that of supersaturation of a
liquid by a salt, and is probably due to some hinderance to change of
molecular movement.
The rate of ordinary chemical corrosion of each metal varied in every
different liquid; in each solution also it differed with every
different metal. The most chemically positive metals were usually the
most quickly corroded, and the corrosion of each metal was usually the
fastest with the most acid solutions. The rate of corrosion at any
given temperature was dependent both upon the nature of the metal and
upon that of the liquid, and was limited by the most feebly active of
the two, usually the electrolyte. The order of rate of corrosion of
metals also differed in every different liquid. The more dissimilar
the chemical characters of two liquids, the more diverse usually was
the order of rapidity of corrosion of a series of metals in them. The
order of rate of simple corrosion in any of the liquids examined
differed from that of chemico-electric and still more from that of
thermo-electric tension. Corrosion is not the cause of thermo-electric
action of metals in liquids.
Out of fifty-eight cases of rise of temperature the rate of ordinary
corrosion was increased in every instance except one, and that was
only a feeble exception--the increase of corrosion from 60° to 160° F.
with different metals was extremely variable, and was from 1.5 to 321.6
times. Whether a metal increased or decreased in thermo-electromotive
force by being heated, it increased in rapidity of corrosion. The
proportions in which the most corroded metal was also the most
thermo-electro-positive one was 65.57 per cent. in liquids at 60° F.,
and 69.12 in the same liquids at 160° F.; and the proportion in which
it was the most chemico-electro-positive at 60 F. was 84.44 per cent,
and at 160° F. 80.77 per cent. The proportion of cases therefore in
which the most chemico-electro-negative metal was the most corroded
one increased from 15.56 to 19.23 per cent, by a rise of temperature
of 100° F. Comparison of these proportions shows that corrosion
usually influenced in a greater degree chemico-electric rather than
thermo-electric actions of metals in liquids. Not only was the
relative number of cases in which the volta-negative metal was the
most corroded increased by rise of temperature, but also the average
relative loss by corrosion of the negative to that of the positive one
was increased from 3.11 to 6.32.
The explanation most consistent with all the various results and
conclusions is a kinetic one: That metals and electrolytes are
throughout their masses in a state of molecular vibration. That the
molecules of those substances, being frictionless bodies in a
frictionless medium, and their motion not being dissipated by
conduction or radiation, continue incessantly in motion until some
cause arises to prevent them. That each metal (or electrolyte), when
unequally heated, has to a certain extent an unlike class of motions
in its differently heated parts, and behaves in those parts somewhat
like two metals (or electrolytes), and those unlike motions are
enabled, through the intermediate conducting portion of the substance,
to render those parts electro-polar. That every different metal and
electrolyte has a different class of motions, and in consequence of
this, they also, by contact alone with each other at the same
temperature, become electro-polar. The molecular motion of each
different substance also increases at a different rate by rise of
temperature.
This theory is equally in agreement with the chemico-electric results.
In accordance with it, when in the case of a metal and an electrolyte,
the two classes of motions are sufficiently unlike, chemical corrosion
of the metal by the liquid takes place, and the voltaic current
originated by inherent molecular motion, under the condition of
contact, is maintained by the portions of motion lost by the metal and
liquid during the act of uniting together. Corrosion therefore is an
effect of molecular motion, and is one of the modes by which that
motion is converted into and produces electric current.
In accordance with this theory, if we take a thermo-electric pair
consisting of a non-corrodible metal and an electrolyte (the two being
already electro-polar by mutual contact), and heat one of their points
of contact, the molecular motions of the heated end of each substance
at the junction are altered; and as thermo-electric energy in such
combinations usually increases by rise of temperature, the metal and
liquid, each singly, usually becomes more electro polar. In such a
case the unequally heated metal behaves to some extent like two
metals, and the unequally heated liquid like two liquids, and so the
thermo-electric pair is like a feeble chemico-electric one of two
metals in two liquids, but without corrosion of either metal. If the
metal and liquid are each, when alone, thermo-electro-positive, and if,
when in contact, the metal increases in positive condition faster than
the liquid by being heated, the latter appears thermo-electro-negative,
but if less rapidly than the liquid, the metal appears
thermo-electro-negative.
As also the proportion of cases is small in which metals that are
positive in the ordinary thermo-electric series of metals only become
negative in the metal and liquid ones (viz., only 73 out of 286 in
weak solutions, and 48 out of the same number in strong ones), we may
conclude that the metals, more frequently than the liquids, have the
greatest thermo-electric influence, and also that the relative
largeness of the number of instances of thermo-electro-positive metals
in the series of metals and liquids, as in the series of metals only,
is partly a consequence of the circumstance that rise of temperature
usually makes substances--metals in particular--electro-positive.
These statements are also consistent with the view that the elementary
substances lose a portion of their molecular activity when they unite
to form acids or salts, and that electrolytes therefore have usually a
less degree of molecular motion than the metals of which they are
partly composed.
The current from a thermo-couple of metal and liquid, therefore, may
be viewed as the united result of difference of molecular motion,
first, of the two junctions, and second, of the two heated (or cooled)
substances; and in all cases, both of thermo- and chemico-electric
action, the immediate true cause of the current is the original
molecular vibrations of the substances, while contact is only a static
permitting condition. Also that while in the case of thermo-electric
action the sustaining cause is molecular motion, supplied by an
external source of heat, in the case of chemico-electric action it is
the motion lost by the metal and liquid when chemically uniting
together. The direction of the current in thermo-electric cases
appears to depend upon which of the two substances composing a
junction increases in molecular activity the fastest by rise of
temperature, or decreases the most rapidly by cooling.
* * * * *