The History of Galvanism divided into six grand Epochs.--Davy
extends the experiment of Nicholson and Carlisle.--His Pile
of one metal and two fluids.--Dr. Wollaston advocates the
doctrine of oxidation being the primary cause of Voltaic
Phenomena.--Davy's modification of that theory.--His Bakerian
Lecture of 1806.--He discovers the sources of the Acid and
Alkaline matter eliminated from water by Voltaic action.--On
the nature of Electrical decomposition and transfer.--On
the relations between the Electrical energies of bodies,
and their Chemical Affinities.--General developement of the
Electro-chemical Laws.--Illustrations, Applications, and
Conclusions.
The History of Galvanism may be divided into six grand epochs; each
being distinguished by the discovery of facts variously interesting
from their novelty, and from the extent and importance of their
applications.
It cannot be expected that I should enter into a minute history of
the science; such a labour would require a distinct work for its
accomplishment. I shall therefore follow the plan of the architect,
who, in presenting a finished drawing of a part, sketches a faint
outline of the whole edifice to which it belongs, in order that its
fair proportions may appear in proper breadth and relief.
The FIRST EPOCH may be considered as arising out of the fundamental
fact discovered by Galvani in 1790--that the contact of two
different metals with the nerve of a recently killed frog will
excite distinct muscular contractions.
THE SECOND EPOCH may be dated from the discovery of what might be
termed _Organic_ Galvanism, or the production of its influence,
without the presence of animal organs, by the peculiar action of
metals upon water, as first observed by Dr. Ash.
THE THIRD EPOCH will long be celebrated on account of the discovery
of the accumulation of the Galvanic power, by the invention of the
pile of Volta, made known in the first year of the present century,
and which so distinctly exhibited the analogy between Galvanism and
Electricity, that the energy thus excited is now generally spoken
of as "_Voltaic Electricity_."
THE FOURTH EPOCH may be considered as founded upon the knowledge of
the general connexion between the excitement of Voltaic electricity
and chemical changes.
THE FIFTH EPOCH is exclusively indebted for its origin to Davy--the
establishment of the general law, that Galvanism decomposes all
compound bodies, and that the decomposition takes place in a
certain determinate manner.
THE SIXTH AND LAST EPOCH is founded upon the discovery of the
relations subsisting between electricity and magnetism; giving
origin to a new branch of science, which has been distinguished by
the name of "ELECTRO-MAGNETISM."
Galvani,[53] from the moment of his first discovery, always
referred the effects he produced to an electrical origin; but he
considered that the metals employed merely acted as conductors,
which effected a communication between the different parts of an
animal, naturally, or by some process of nature, in opposite states
of electricity, and that the muscular contractions took place
during the restoration of the equilibrium.
[53] The simple fact relating to the action of metals on the
animal organs was certainly not first observed by Galvani, but by
Sulzer, who has described the sensation of taste produced by the
contact of lead and silver with the tongue, in his _Théorie des
Plaisirs_, in 1767.
Until the researches of Dr. Ash,[54] Ritter, Fabroni, and Creve,
had been made known, the Galvanic influence was generally
considered as existing only in the living organs of animals, from
which it might be elicited by certain processes.
[54] M. Humboldt (_Ueber die gereize Faser_, l. 473, 1797,)
quotes part of a letter from Dr. Ash, in which it is said that,
"if two finely polished plates of homogeneous zinc be moistened
and laid together, little effect follows; but if zinc and silver
be tried in the same way, the whole surface of the silver
will be covered with oxidated zinc. Lead and quicksilver act
as powerfully upon each other, and so do iron and copper." M.
Humboldt says, that, in repeating this experiment, he saw air
bubbles ascend, which he supposes to have been hydrogen gas from
the decomposition of water.
In the Bakerian Lecture[55] read before the Royal Society in
1826, Davy, in giving a retrospective view of the progress of
Electro-chemical Science, very justly remarks, that the true
origin of all that has been done in this department of philosophy
was the accidental discovery of Nicholson and Carlisle, of the
decomposition of water by the pile of Volta, on the 30th of April,
in the year 1800; which was immediately followed by that of the
decomposition of certain metallic solutions, and by the observation
of the separation of alkali on the negative plates of the
apparatus. Mr. Cruickshank, in pursuing these experiments, obtained
many new and important results, such as the _decomposition of the
muriates of magnesia_, _soda_, and _ammonia_; and also observed the
fact, that alkaline matter always appeared at the negative, and
acid matter at the positive pole.[56]
[55] As this lecture will be frequently mentioned in the progress
of these Memoirs, in connexion with most important discoveries,
it may be interesting to the reader to learn something of its
foundation and design. I have therefore collected the necessary
information from the Minutes of the Royal Society. Mr. Baker is
well known in the history of Science, as an accurate observer
with the microscope, and as the author of several works on the
subject. By his will, dated July 1763, he bequeathed the sum
of one hundred pounds, the interest of which he directed "to
be applied for an Oration or Discourse, to be read or spoken
yearly by some one of the Fellows of the Royal Society, on such
parts of Natural History, or Experimental Philosophy, at such
time, and in such manner, as the President and Council of the
said Society shall please to order and appoint; on condition,
nevertheless, that if any one year shall pass after the payment
of the said hundred pounds, without such oration or discourse
having been read or spoken at some Meeting of the said Royal
Society, the said hundred pounds shall then become forfeited, and
shall be repaid by the said Society to his executors," &c. Baker
died in November 1774, and in the following year a Fellow was
nominated to read the lecture. It is a whimsical circumstance,
that the first lecturer should have been PETER WOULFE, the last
of the alchemists. The names of the successive lecturers were as
follow:--Dr. Ingenhouz, Mr. Cavallo, Mr. Vince, Dr. Wollaston,
Dr. Young, Sir H. Davy, Mr. Brande, Captain Kater, Captain Edward
Sabine, and Mr. Herschel.
[56] Nicholson's Journal, vol. iv. p. 190.
No sooner had Davy become acquainted with the curious experiments
of Nicholson and Carlisle, than, as we learn from his letter to
Mr. Gilbert,[57] bearing the date of July 1800, he proceeded to
repeat them. Indeed, it was the early habit of his mind not only to
originate enquiries, but without delay to examine the novel results
of other philosophers; and in numerous instances it would seem,
that he only required to confirm their accuracy before he succeeded
in rendering the application of them subservient to farther
discovery. This was certainly the case with respect to the subject
before us: he was a discoverer as soon as he became an enquirer. It
is admirable to observe with what a quick perception he discovered
the various bearings of a new fact, and with what ingenuity
he appropriated it for the explanation of previously obscure
phenomena. In referring to the "Additional Observations" appended
to his "Chemical Researches," we shall find that the moment he
became acquainted with the experiments of Dr. Ash, he proceeded to
enquire how far the fact, previously noticed by himself, of the
conversion of nitrous _gas_ into nitrous _oxide_, by exposure to
wetted zinc, might depend upon galvanic action.
[57] See page 85.
In the month of September 1800, he published his first paper on the
subject of Galvanic Electricity, in Nicholson's Journal, which was
followed by six others, in which he so far extended the original
experiment of Nicholson and Carlisle, as to show that oxygen and
hydrogen might be evolved from separate portions of water, though
vegetable and even animal substances intervened; and conceiving
that all decompositions might be _polar_, he electrized different
compounds at the different extremities, and found that sulphur
and metallic bodies appeared at the _negative_ pole, and oxygen
and azote at the _positive_ pole, though the bodies furnishing
them were separated from each other. Here was the dawn of the
Electro-chemical theory.
In a letter to Mr. Gilbert, already printed in these Memoirs,[58]
he announced his opinion that Galvanism is a process principally
chemical; and in a subsequent communication[59] to the same
gentleman, written on the eve of his departure from Bristol to the
Royal Institution, we discover a farther developement of the same
theory, which, although modified by future researches, became, as
we shall hereafter find, materially instrumental in establishing
juster views of the nature of Voltaic action.
[58] See page 110.
[59] Page 118.
As soon as it was discovered that galvanic power might be excited
by the contact of metals, without the interposition of animal
organs, it was imagined that the electricity was set in motion
by the contact of bodies possessing different conducting powers,
without any reference to the chemical action which accompanied the
process. This theory was naturally suggested by the fact discovered
by Mr. Bennett several years before--that _electricity is excited
by the mere contact of different metals_: thus, when a plate of
copper and another of zinc, each furnished with an insulating
glass handle, are made to touch by their flat surfaces, the zinc,
after separation, exhibits _positive_, and the copper _negative_
electricity. In this case, it is fair to conclude that a certain
quantity of electricity had moved from the copper to the zinc.
On trying other metals, Volta found that similar phenomena arose;
from which property such bodies have been denominated "_motors_" of
electricity, and the process which takes place _electro-motion_:
terms which have since been sanctioned and adopted by Davy.
It is on this transference of electricity from one surface to
another, by simple contact, that Volta explains the action of
the pile invented by himself, as well as that of all similar
arrangements. The interposed fluids, on this hypothesis, have no
effect as chemical agents, in producing the phenomena; they merely
act as conductors of the electricity.
We have seen how early Davy had observed the intimate connexion
subsisting between the electrical effect, and the chemical changes
going on in the pile, and that he accordingly drew the conclusion
of the dependence of the one upon the other. In fact, the most
powerful Voltaic combinations are those formed by substances that
act chemically upon each other with the greatest energy; while
such as undergo no chemical change exhibit no electrical powers:
thus zinc, copper, and nitric acid form a powerful battery; whilst
silver, gold, and water, which do not act upon each other, produce
no sensible effect in a series of the same number.
Although, in this obscure region of research, we are as yet unable
to discover the nature of the power by which electricity is
accumulated, it was a considerable step towards a true theory to
have ascertained the insufficiency of the proposition that had
been offered in explanation of the phenomena.
An investigation into the chemical activity of the pile led Davy
to the discovery of a new series of facts, to which we find
an allusion in his former letters to Mr. Gilbert, and which
subsequently formed the basis of his first communication read
before the Royal Society on the 18th of June in the same year.
All the combinations analogous to the Voltaic pile had hitherto
consisted of a series containing, at least, _two_ metallic
bodies, (or one metal and charcoal,) and a stratum of fluid. Davy
discovered that an accumulation of galvanic energy, exactly similar
to that in the common pile, might be produced by the arrangement
of _single_ metallic plates with _different_ strata of fluids; so
that, instead of composing a battery with _two_ metals and _one_
fluid, he succeeded in constructing it with _one_ metal and _two_
fluids; provided always that oxidation, or some equivalent chemical
change, should proceed on one of the metallic surfaces only.
In describing these combinations of a single metal with two fluids,
he divides them into three classes, following in the arrangement
the order of time with regard to their discovery.
In the First Class, one side of the metallic plate is oxidated;
in the Second, a sulphuret is formed on one of its surfaces; and
in the Third, both sides are acted upon, the metal becoming a
_sulphuret_ on one of its surfaces, and an _oxide_ on the other.
The apparatus which he employed for these experiments is preserved
in the laboratory of the Royal Institution. It consists of a
trough, containing grooves capable of receiving the edges of the
different plates necessary for the arrangement, one half of which
are composed of horn, the other half of some one metal.
When the apparatus was used, the cells were filled, in the galvanic
order, with the different solutions, according to the class of the
combination, and connected in pairs with each other by slips of
moistened cloth carried over the non-conducting plates.
At the meeting of the Royal Society, following that on which the
above interesting facts were communicated, Dr. Wollaston presented
a memoir of considerable importance, entitled, "Experiments on
the Chemical Production and agency of Electricity;" in which he
strongly advocates the truth of that theory which recognises
metallic oxidation as the _primary_ cause of the Voltaic phenomena.
This paper is also farther important as it proves, by most
ingeniously devised experiments, not only the similarity of the
means by which both common and galvanic electricity are excited,
but also the resemblance existing between their effects; showing,
in fact, that they are both essentially the same, and confirming
the opinion, that all the apparent differences may depend upon
differences in intensity and quantity.
Acting upon this principle, Dr. Wollaston succeeded in producing a
very close imitation of the chemical action of galvanism by common
electricity; such, for example, as the decomposition of water, and
other effects of oxidation and deoxidation.[60] In the prosecution
of this train of research, he displays, in a very striking
manner, that attention to minute arrangement which so remarkably
characterised all his manipulations. I particularly allude to
the expedients by which he reduced the extremity of a gold wire,
in order to apportion the strength of the electric charge to the
quantity of water submitted to its influence.
[60] M. Bonijol of Geneva has lately succeeded in effecting
the decomposition of _Potash_ and the _Chloride of Silver_ by
ordinary electricity. His process consists in placing these
substances in a very narrow glass tube, and in then passing a
series of electric sparks from the ordinary machine through
them. The electricity was conducted into the tube by means of
two metallic wires fixed into the ends. When a quick succession
of electric sparks had taken place for about five or ten
minutes, the tube containing chloride of silver was found to
contain reduced silver; and when potash had been submitted to
the electric current, then the Potassium was seen to take fire
as it was produced. The same philosopher has likewise contrived
to decompose water by atmospheric electricity. The electricity,
in this case, is collected from the atmosphere by means of a
very fine point fixed at the extremity of an insulated rod; the
latter is connected with the apparatus, in which the water is to
be decomposed, by a metallic wire, of which the diameter does
not exceed 1-50th of an inch. In this way the decomposition of
the water proceeds in a continuous and rapid manner, although
the atmospheric electricity be not strong. Stormy weather, it is
said, is quite sufficient for the purpose.--_Bib. Univers._ 1830,
p. 213. and _Royal Institution Journal_, No. 2.
Although it is now very generally admitted, that the chemical
agency of the fluids upon the metals employed is highly essential
to the maintenance of Voltaic action, there still remains
considerable doubt as to how far we are entitled to regard it as
the first in the order of phenomena.
At a later period of his researches, Davy suggested as a correction,
or rather modification, of the theory of Volta, that the
electro-motion produced by the contact of the metals might
be the primary cause of the chemical changes; and that such
changes were in no other way efficient, than in restoring the
electric equilibrium thus disturbed: it was farther held, that
this equilibrium could not be permanent, that it could in fact be
only momentary; since, in consequence of the imperfect conducting
power of the interposed fluid, the zinc and copper-plates, by their
electro-motive power, would again assume their opposite states
of electricity; and that these alternate changes would occur,
as long as any of the fluid remained undecomposed. In a Voltaic
arrangement, then, there would appear to exist, if the expression
may be allowed, a kind of electrical seesaw; the apposition of
the metals destroying the equilibrium, and the resulting chemical
changes again restoring it. It has, however, been very justly
observed, that the application of electricity, as an instrument
of chemical decomposition, has most fortunately no connexion with
such theories, and that the study of its effects may be carried on
without reference to any hypothetical notions concerning the origin
of the phenomena.
An interval of nearly five years had elapsed between the first
communication which Davy made on this subject, and the Bakerian
lecture which is immediately to be considered. During this period
several new facts had been added by different experimentalists, but
they were scattered, disjointed, and totally unconnected with each
other by any rational analogies.
The constant appearance of acid and alkaline matter in pure water,
when submitted to the influence of the Voltaic pile, gave rise
to the most extravagant speculations and discordant hypotheses.
Various statements were made, both in Italy and England, respecting
the _generation_ of muriatic acid, and that of the fixed alkalies,
under these circumstances. Mr. Sylvester affirmed, that if two
separate portions of water were electrised out of the contact of
substances containing alkaline or acid matter, acid and alkali
would, nevertheless, be produced.
Some philosophers sought to explain the phenomenon from the salts
contained in the fluids of the trough, which they imagined might,
by some unsuspected channel, find their way into the water under
examination. Others believed that they were actually _generated_
by the union of the electric fluid with the water, or with one or
both of its elements; so that, up to the time of Davy's masterly
researches, the subject was involved in the greatest obscurity; and
whether the saline matter was liberated from unknown combinations,
or at once formed by the union of its elements, was a question upon
which the greatest chemists entertained different opinions.
The Bakerian Lecture, read before the Royal Society on the 20th of
November 1806, not only set this question for ever at rest, but
unfolded the mysteries of general Voltaic action; and, as far as
theory goes, may almost be said to have perfected our knowledge of
the chemical agencies of the pile.
This grand display of scientific light burst upon Europe like a
splendid meteor, throwing its radiance into the deepest recesses,
and opening to the view of the philosopher new and unexpected
regions.
I shall endeavour to offer as popular a review of this celebrated
memoir, as the abstruse and complicated nature of its subjects will
allow; and I shall be careful in pointing out the successive stages
of the enquiry; for we are all too much in the habit of exclusively
looking after results; whereas an examination of the steps by which
they were attained is far more important, not only to the fame of
the discoverer, but to ourselves, as the means of instruction.
The subjects investigated in this memoir are arranged under the
following divisions.
1. "On the changes produced in Water by Electricity.
2. "On the agencies of Electricity in the decomposition of various
compound Bodies.
3. "On the transfer of certain constituent Parts of Bodies by the
action of Electricity.
4. "On the passage of Acids, Alkalies, and other Substances,
through various attracting chemical menstrua, by means of
Electricity.
5. "Some general Observations on these Phenomena, and on the mode
of Decomposition and Transition.
6. "On the General Principles of the chemical changes produced by
Electricity.
7. "On the Relations between the Electrical Energies of bodies and
their Chemical Affinities.
8. "On the mode of action of the Pile of Volta, with Experimental
Elucidations.
9. "On some General Illustrations and Applications of the foregoing
facts and principles."
With respect to the first of these divisions, comprehending a
history of the changes produced in water by electricity, it is
worthy of particular notice, that as early as the year 1800,
while residing at Bristol, Davy had discovered that when separate
portions of distilled water, filling two glass tubes connected by
moist bladders, or any moist animal or vegetable substance, were
submitted to the electrical action of the Voltaic pile, by means
of gold wires, a _nitro-muriatic_ solution of gold appeared in the
tube containing the positive wire, and a solution of soda in the
opposite tube; but he soon ascertained that the muriatic acid owed
its appearance to the animal or vegetable matters employed; for
when the same fibres of cotton were used in successive experiments,
and washed after every process in a weak solution of nitric acid,
the water in the apparatus containing them, though acted upon for a
great length of time with a very strong power, produced no effect
upon a solution of nitrate of silver.
In every case in which he had procured much soda, the glass[61] at
the point of contact with the wire seemed considerably eroded; when
by substituting an agate for a glass cup, no fixed saline matter
could be obtained. Its source therefore, in the former case, was
evidently the glass.
[61] It is perhaps a fact not very generally known, that glass,
to a certain extent, is decomposable by water: if some of it in
a powdered state be triturated with distilled water, in a short
time the turmeric test will indicate a portion of alkali in
solution.
With respect to Mr. Sylvester's experiment, already noticed, it
was sufficient to say that he conducted his process in a vessel of
_pipe-clay_, which not only contains lime, but may also include in
its composition some of the combinations of a fixed alkali.
On resuming the enquiry, it was Davy's first care to remove
every possible source of impurity: he accordingly procured cups
of agate, which, previously to being filled, were boiled for
several hours in distilled water; and a piece of very white and
transparent _amianthus_, a substance first proposed for this
purpose by Dr. Wollaston, having been similarly purified, was made
to connect the vessels together. Thus was every apparent source of
fallacy removed; but still, after having been exposed to Voltaic
action for forty-eight hours, the water in the positive cup gave
indications of muriatic acid, and that in the negative cup, of
soda! The result was as embarrassing as it was unexpected; but it
was far from convincing him that the bodies thus obtained were
_generated_:--but whence arose the saline matter? Did the agate,
after every precaution, still contain some very minute portion
of saline matter, not easily discoverable by chemical tests? To
determine this question, the experiment was repeated a second,
a third, and a fourth time: the quantities of saline matter
diminished in every successive operation, which sufficiently proved
that the agate must at least have been _one_ of the sources sought
for; but four additional repetitions of the process convinced
the operator that it could not be the only one; that there must
exist some other source from which the alkali proceeded, since
it continued to appear to the last, in quantities sufficiently
distinct, and apparently equal, in every experiment. This was
extremely perplexing: every precaution had been taken--the agate
cups had even been included in glass vessels, out of the reach of
the circulating air--all the acting materials had been repeatedly
washed with distilled water; and no part of them in contact with
the fluid had ever touched the fingers.
The water itself then, however pure it might appear, must have
furnished the alkali. The experiments were repeated in cones of
the purest gold, and the water contained in them was submitted to
Voltaic action for fourteen hours; the result was, that the acid
increased in quantity as the experiment proceeded, and at length
became even sour to the taste. On the contrary, the alkaline
properties of the fluid in the opposite cone shortly obtained a
certain intensity, and remained stationary.
On the application of heat, the alkaline indications became less
vivid, although there always remained, after the operation,
sufficient evidence to prove that a portion at least was fixed,
although probably mixed with ammonia.
The acid, as far as its properties could be examined, agreed with
those of pure nitrous acid, having an excess of nitrous gas.
It was now impossible to doubt that the water held in solution some
substance which was capable of yielding alkaline matter, but which,
from the minuteness of its quantity, had soon been exhausted.
The next step, therefore, was to submit the water to a still more
rigorous examination, which he did by evaporating it in a vessel of
silver; when he had the satisfaction to discover the 1-70th of a
grain of saline matter.
The water, thus purified in a vessel of silver, was again subjected
to Voltaic action in the cones of gold. After two hours, there was
only the slightest possible indication of alkali; and this was not,
as before, _fixed_, but entirely _volatile_.
In every one of these experiments, acid matter had been produced,
and it always presented the character of nitrous acid. Two of the
great sources of foreign matter had been detected and removed, viz.
the vessels, and the water employed; it still however remained to
be explained, how nitrous acid and ammonia could be produced in
cases where pure water and pure vessels had been used. In no part
of this elaborate enquiry is the penetration of Davy more striking,
than in his reasonings upon this problem, and in the beautiful
experiments which his sagacity suggested for its solution.
It occurred to him, that the nascent oxygen and hydrogen of the
water might respectively combine with a portion of the nitrogen
of the common air, which is constantly dissolved in that fluid;
but if this were the case, how did it happen that the production
of nitrous acid was progressive, while that of the alkali was
limited? The experiments of Dr. Priestley, on the absorption of
gases by water, at once suggested themselves to his mind as being
capable of solving this last difficulty; for that distinguished
philosopher had shown, that hydrogen, during its solution in
water, expelled the nitrogen, whereas oxygen and nitrogen were
capable of coexisting in a state of solution in that fluid. It was,
however, necessary to confirm the truth of this explanation by
experiment, and he accordingly introduced the two cones of gold,
containing purified water, under the receiver of an air-pump; the
exhaustion was effected, and the Voltaic pile brought to act upon
the water thus circumstanced; after eighteen hours the result was
examined, when the water in the negative cone produced no effect
upon prepared litmus, but that in the positive vessel did give it a
tinge of red barely perceptible.
Had his series of experiments terminated here, the truth of his
conclusions would have been established by the comparatively
small proportion of acid formed in this latter experiment; but he
determined to repeat it under circumstances, if possible, still
more unexceptionable and conclusive. Having, therefore, arranged
the apparatus as before, he exhausted the receiver, and then filled
it with hydrogen gas from a convenient air-holder; he made even a
second exhaustion, to ensure the highest accuracy, and then again
introduced carefully prepared hydrogen. The Voltaic process was
continued during twenty-four hours, and at the end of that period
it was found that neither the water in the positive nor in the
negative vessels altered the tint of litmus in the slightest degree.
Thus did he succeed in exposing the three great sources of fallacy
which had so long misled chemists, with regard to the generation
of acid and alkaline matter in Voltaic experiments, viz.--The
impurities of the vessels--the foreign matter contained in the
water--and the compounds generated by the combination of the
nitrogen of atmospheric air with the elements evolved from water;
and thus did he establish, by an unbroken chain of incontrovertible
evidence, the important truth, that "water, chemically pure, is
decomposed by electricity into gaseous matter alone--into oxygen
and hydrogen."
Out of the foregoing train of research very naturally sprang the
consideration of the _decomposing agencies of Electricity_. It had
been constantly observed, that, in all electrical changes connected
with the presence of acid and alkaline matter, the former uniformly
collected around the positive, and the latter around the negative
surface of the apparatus.
In one of the earliest experiments, Davy had also noticed that
glass underwent decomposition, and that its alkali always passed to
the negative surface. He was, therefore, led to enquire whether,
through electrical agency, different solid earthy compounds,
insoluble, or soluble with difficulty in water, might not be made
to undergo a similar decomposition. We shall find that the results
of the trials were decisive and satisfactory. For conducting
experiments of this description, he hit upon the happy expedient
of constructing the cups with the materials which he wished to
submit to experiment, and then by introducing water into them, and
forming the necessary connexion by means of asbestus, he completed
the Voltaic circuit. In this manner he submitted to experiment
_sulphate of lime_, _sulphate of strontia_, _fluate of lime_,
_sulphate of baryta_, &c. and with analogous results; the acid
element in each case passing to the positive, and the earthy base
to the negative cup.
As, in the above experiments, the bodies under examination were
presented in considerable masses, and exposed large surfaces to
the electric action, it became necessary to enquire whether minute
portions of acid and alkaline matter could, by the same agency, be
disengaged from solid combinations. This point was very readily
elucidated. A piece of fine grained basalt, which, by a previous
analysis, had been found to contain 3·5 per cent. of soda, nearly
·5 of muriatic acid, and fifteen parts of lime, having been
divided into two properly-shaped pieces, and a cavity, capable
of containing twelve grains of water, been drilled in each, was
submitted, as in former experiments, to the action of the pile. At
the end of ten hours, the result was examined with care, when it
appeared that the positively electrified water had the strong smell
of oxymuriatic acid, and copiously precipitated nitrate of silver;
while that which was negative affected turmeric, and left by
evaporation a residuum which appeared to consist of lime and soda.
A part of a specimen of compact zeolite from the Giants' Causeway,
and vitreous lava from Ætna, were each treated in a similar manner,
and with results equally satisfactory.
Having thus settled the question with regard to the disengagement
of the saline parts of bodies insoluble in water, he proceeded
to extend and multiply his experiments on soluble compounds,
the decomposition of which, as might have been supposed, always
proceeded with greater rapidity, and furnished results more
perfectly distinct. In these processes he employed the agate cups,
with platina wires, connected by amianthus moistened with pure
water; the solutions were introduced into these cups, and the
electrifying power applied in the manner already described. In this
way, _sulphate of potash_, _sulphate of soda_, _nitrate of potash_,
_phosphate of soda_, &c. were respectively examined; and in every
case the acid, after a certain interval, collected in the cup
containing the positive wire, and the alkalies and earths in that
containing the negative wire.
When metallic solutions were employed, metallic crystals or
depositions were formed on the negative wire, and oxide was
likewise deposited around it, while a great excess of acid was
found in the opposite cup.
With respect to the transfer of the constituent Parts of Bodies by
Electric Action, several original experiments were instituted, and
some important conclusions established.
Several facts had been stated, which rendered it probable that
the saline elements evolved in decompositions by electricity,
were capable of being transferred from one electrified surface to
another, according to their usual order of arrangement; but to
demonstrate this clearly, farther researches were required, and
Davy proceeded to supply the necessary evidence. He connected one
of the cups of sulphate of lime before mentioned, with a cup of
agate, by means of asbestus, and filling them with purified water,
connected them with the battery. In about four hours, a strong
solution of lime was found in the agate cup, and sulphuric acid in
the cup of sulphate of lime. By reversing the order of arrangement,
and carrying on the process during a similar period, the sulphuric
acid appeared in the agate cup, and the lime in the opposite
vessel. In both these experiments (the acid in the one case, and
the lime in the other), the elements of the substance must have
passed, in an imperceptible form, along the connecting line of
asbestus into the opposite vessel.
Many trials were made with other saline bodies, and with results
equally satisfactory; the base always passing into the vessel
rendered negative, and the acid into that which was positive.
The time required for these transmissions appeared to be, _cæteris
paribus_, in some proportion to the length of the intermediate
volume of water.
In the farther prosecution of the enquiry, Davy discovered a still
more extraordinary series of facts. In the first place, he found
that the contact of the saline solution with a metallic surface was
not in the least necessary for its decomposition. He introduced
purified water into two glass tubes, and connected with them, by
means of amianthus, a vessel containing a solution of muriate of
potash. In this case, the saline matter was distant from each of
the wires at least two-thirds of an inch; and yet alkaline matter
soon appeared in one tube, and acid matter in the other; and in
sixteen hours moderately strong solutions of potash and muriatic
acid had been formed.
The discovery of this fact became the key to that of others. He
very naturally proceeded to enquire into the progress of the
transfer, and into the course of the acid and alkaline elements;
when, by the use of litmus and turmeric, he arrived at the
following conclusion,--that acids and alkalies, during their
electrical transference, passed through water containing vegetable
colours without effecting in them any change. From which we are led
to the consideration of the fourth division of the subject, viz.
"On the Passage of Acids, Alkalies, and other Substances, through
various attracting Chemical Menstrua, by Electricity."
As soon as it was discovered that a power generated by the Voltaic
pile was capable of destroying elective affinity in the vicinity
of the metallic points, it seemed reasonable to suppose, that
the same power might also destroy it, or at least suspend its
operation, throughout the whole of the circuit. The truth of such
a supposition was at once placed beyond all doubt by the following
very striking experiment.
Three tubes, the first containing a solution of _sulphate of
potash_, the second a weak solution of _ammonia_, and the third,
_pure water_, each being connected with the other in the usual
manner by amianthus, were arranged in relation to the pile, as
follow:--the _sulphate of potash_ was placed in contact with the
negatively electrified point, the _pure water_ with the positively
electrified point, while the solution of _ammonia_ was made the
middle link of the conducting chain; so that no sulphuric acid
could pass to the positive point in the distilled water, without
passing through the ammoniacal solution.
In less than five minutes after the electric current had been
completed, it was found, by means of litmus paper, that acid was
in the act of collecting around the positive point; and in half an
hour the result was sufficiently distinct for accurate examination.
Other experiments were made with a solution of lime, and with weak
solutions of potash and soda, and the results were analogous.
Muriatic acid, from muriate of soda, and nitric acid, from nitrate
of potash, were also transmitted through concentrated alkaline
menstrua, under similar circumstances, and with like effects.
Davy also made several experiments on the transition of alkaline
and acid matter, through different neutro-saline solutions, the
results of which were exactly such as theory would have anticipated.
In conducting, however, these experiments of electrical
transference, there would appear to be one condition essential to
their success, viz. that the solution contained in the intermediate
vessel should not be capable of forming an insoluble compound with
the substance transmitted through it: thus, for example, Davy
found that _strontia_ and _baryta_ passed, like the other alkaline
substances, very readily through muriatic and nitric acids;
and _vice versâ_, that these acids passed with equal facility
through aqueous solutions of the earths in question; but when it
was attempted to pass _sulphuric_ acid through the same earthy
solutions, or to pass the earths through the sulphuric acid, that
then the results were of a very different character: the sulphuric
acid, in its passage through the barytic solution, was arrested in
its progress by the earthy body, and falling down as an insoluble
compound with it, was carried out of the sphere of the electrical
action, by which the power of transfer was destroyed. The same
phenomena occurred whenever he attempted to pass muriatic acid
through a solution of sulphate of silver. We now come to the next
division--viz. "Some general Observations on these Phenomena, and
on the mode of Decomposition and Transition."
Davy considers that it will be a general expression of the facts
relating to the changes and transitions by electricity, to say,
that "hydrogen, the alkaline substances, the metals, and certain
oxides, are attracted by negatively electrified, and repelled by
positively electrified metallic surfaces; and on the contrary, that
oxygen and acid substances are attracted by positively electrified,
and repelled by negatively electrified metallic surfaces."
And moreover, that these "attractive and repulsive forces are
sufficiently energetic to destroy or suspend the usual operation of
elective affinity."
Amidst all these wonderful phenomena, that perhaps which excites
our greatest astonishment is the fact of the transfer of ponderable
matter to a considerable distance, through intervening substances,
and in a form that escapes the cognizance of our senses! Upon
this question, Davy offers the following remarks:--"It is," says
he, "very natural to suppose, that the repellent and attractive
energies are communicated from one particle to another particle of
the same kind, so as to establish a conducting chain in the fluid;
and that the locomotion takes place in consequence: thus, in all
the instances in which I examined alkaline solutions through which
acids had been transmitted, I always found acid in them, as long
as any acid matter remained at the original source. In time, by
the attractive power of the positive surface, the decomposition
and transfer undoubtedly become complete; but this does not affect
the conclusion. In cases of the separation of the constituents of
water, and of solutions of neutral salts forming the whole of the
chain, there may possibly be a succession of decompositions and
recompositions throughout the fluid."
We are next brought to a very important point in the enquiry--viz.
"The consideration of the General Principles of the chemical
changes produced by Electricity."
The experiment of Mr. Bennett, already alluded to, had shown that
many bodies, when brought into contact, and afterwards separated
from each other, exhibited signs of opposite states of electricity:
but it is to the investigations of M. Volta that we are indebted
for the clear developement of the fact; for he has distinctly
proved it in the case of copper and zinc, and other metallic
combinations, and he supposed that it might also take place with
regard to metals and fluids.
In a series of experiments, made in the year 1801, on the
construction of electrical combinations, by means of alternations
of single metallic plates, and different strata of fluids, as
explained upon a former occasion,[62] Davy had observed that, when
acid and alkaline solutions were employed as the elements of these
Voltaic combinations, the alkaline solutions always received
the electricity from, and the acid always transmitted it to the
metal. These principles seem to bear an immediate relation to those
general phenomena of decomposition and transfer, which have been
the subject of the preceding details.
[62] Page 223.
In the most simple case of electrical action, the alkali which
receives electricity from the metal would necessarily, on being
separated from it, appear _positive_; whilst the acid, under
similar circumstances, would be _negative_; and these bodies having
respectively, with regard to the metal, that which may be called a
positive and a negative electrical energy, in their repellent and
attractive functions, would seem to be governed by the common laws
of electrical attraction and repulsion; the body possessing the
positive energy being repelled by positively electrified surfaces,
and that possessing the negative influence following the contrary
order.
Davy made a number of experiments with the view of elucidating this
idea, and of extending its application; and, in all cases, their
results tended, in a most remarkable manner, to confirm the analogy.
He proceeded, by means of very delicate instruments, to ascertain
the electrical states of single insulated acid and alkaline
solutions, after their contact with metals; but the sources of
errors were so numerous, as to render the results far from being
satisfactory; but in experiments on dry and solid bodies, the
embarrassments arising from evaporation, chemical action, &c. did
not occur. When perfectly dry oxalic, succinic, benzoic, or boracic
acid, either in the form of powder or crystals, were touched upon
an extended surface with a plate of copper, insulated by a glass
handle, the copper was found positive, the acid negative. When
again metallic plates were made to touch dry lime, strontia, or
magnesia, they became negative: in these latter experiments the
effect was exceedingly satisfactory and distinct; a single contact
upon a large surface being sufficient to communicate a considerable
charge.
Numerous other trials were made, and the results confirmed the
principle; and moreover proved, as might have been expected, that
bodies possessing electrical conditions with regard to one and the
same body, possessed them with regard to each other: for instance,
a dry piece of lime became positively electrical by repeated
contact with crystals of oxalic acid.
These results led him to reason more fully upon the "Relations
between the Electrical energies of bodies and their Chemical
affinities."
As the chemical attraction subsisting between two bodies seems
to be destroyed by giving to one of them an electrical condition
opposite to that which it naturally possesses; and since the
substances that combine chemically, as far as can be ascertained,
exhibit opposite states of electricity, the relations between
this energy and chemical affinity would appear to be sufficiently
evident to warrant the conclusion at which Davy arrived, viz. that
"the combinations and decompositions by electricity were referable
to the law of electrical attractions and repulsions;" from which
he advanced to the still more important step--"that chemical and
electrical attractions were produced by the same cause, acting in
one case on particles, in the other on masses."
From these views, he is led to propose the electrical powers,
or the forces required to disunite the elements of bodies, as
a test or measure of the intensity of chemical attraction. An
accurate investigation into this connexion, which may be called the
_Electro-dynamic_ relations of bodies to their combining masses or
proportional numbers, would be the first step towards fixing the
science of Chemistry on the permanent foundation of the Mathematics.
If, then, the power of electrical attraction and repulsion be
identified with chemical affinity, or rather, if both be dependent
upon the same cause, it will follow that two bodies which are
naturally in opposite electrical states, may have these states
sufficiently exalted to give them an attractive power superior to
the cohesive force opposed to their union; when a combination will
take place which will be more or less energetic, as the opposed
forces are more or less equally balanced. Again, when two bodies,
repellent of each other, act upon a third with different degrees
of the same electrical energy, the combination will be determined
by the degree; or, if bodies having different degrees of the same
electrical energy with respect to a third, have likewise different
energies with respect to each other, there may be such a balance of
attracting and repelling forces as to produce a triple compound;
and by the extension of this reasoning, complicated chemical union
may be easily explained.
Whenever bodies brought by artificial means into a high state of
opposite electricities are made to restore the equilibrium, heat
and light are the common consequences. It is perhaps an additional
circumstance in favour of the theory to state, that heat and light
are likewise the results of all intense chemical action. And as
in certain forms of the Voltaic battery, where large quantities
of electricity of low intensity act, heat without light is
produced; so in slow chemical combinations there is an increase of
temperature without any luminous appearance.
The effect of heat in producing combination may be easily explained
according to these ideas; it not only gives more freedom of motion
to the particles, but in a number of cases it seems to exalt the
electrical energies of bodies:--glass, the tourmaline, sulphur, and
some others, afford familiar instances of this latter species of
energy.
In general, when the different energies are strong and in perfect
equilibrium, the combination ought to be quick, the heat and light
intense, and the new compound in a neutral state. This would seem
to be the case in the combination of oxygen and hydrogen, which
form water, a body apparently neutral in electrical energy to most
others; and also in the circumstances of the union of the strong
alkalies and acids. But where one energy is feeble, and the other
strong, all the effects must be less vivid; and the compound,
instead of being neutral, ought to exhibit the excess of the
stronger energy.
The grand principle thus developed may enable us to obtain
new and useful indications of the composition of bodies, by
ascertaining the character of their electrical energies; and we
now find, in most modern works of Chemistry, that bodies are
arranged according to their natural electrical relations; and are
said to be ELECTRO-POSITIVE, or ELECTRO-NEGATIVE, according to
their polarities. The advantage of such an arrangement must be
freely acknowledged, for it has been the means of establishing
analogies[63] of the utmost importance in chemistry, of which I
shall adduce some striking examples in a subsequent part of the
present work, when I shall endeavour to offer a general view of
the revolution which chemical science has undergone during the
investigations of Davy, and contemporary philosophers.
[63] It will be sufficient for my present purpose to point out
those existing between _Chlorine_, _Iodine_, and _Bromine_.
After some further enquiries into the theory of the Voltaic
pile,[64] to which an allusion has been already made, the author
offers additional reasons for supposing the decomposition of the
chemical menstrua essential to the continued electro-motion of the
pile; and if the fluid medium could be a substance incapable of
decomposition, there is every reason to believe the equilibrium
would be restored, and the motion of the Electricity cease. Having
shown the effects of _induction_, in increasing the electricity
of the opposite plates, he arrives at the important conclusion,
that in a Voltaic arrangement the _intensity of the Electricity
increases with the number, but the quantity with the size of
the plates_. A theory which was subsequently confirmed by the
experiments of Mr. Children.
[64] See page 226.
The paper concludes with "some general illustrations and
applications of the foregoing facts and principles," and which
the author thinks will readily suggest themselves to the
philosophical enquirer. They offer, for instance, very easy
methods of separating acid and alkaline matter, where they exist
in combination in mineral substances; and, in like manner, they
suggest the application of electrical powers for effecting the
decomposition of animal and vegetable bodies.
On exposing a piece of muscular fibre to the action of the battery,
he found that potash, soda, ammonia, lime, and oxide of iron, were
evolved on the negative side, and the three mineral, together with
the phosphoric, acids, were given out on the positive side.
A laurel leaf, similarly treated, yielded to the negative vessel
resin, alkali, and lime; while in the positive one there collected
a clear fluid, which had the smell of peach-blossoms, and which,
when neutralized by potash, gave a blue-green precipitate to a
solution of sulphate of iron; so that it must have contained
_Prussic Acid_.
A small plant of mint, in a state of healthy vegetation, on being
made the medium of connection in the battery, yielded potash
and lime to the water negatively electrified, and acid to that
positively electrified. The plant recovered after the process; but
a similar one, that had been electrified during a longer period,
faded and died.
These facts would seem to show, that the electrical powers of
decomposition even act upon vegetable matter in its living
condition; and phenomena are not wanting to show that they operate
also on the system of living animals. When the fingers, after
having been carefully washed with pure water, are brought in
contact with this fluid in the positive part of the circuit, acid
matter is rapidly developed, having the character of a mixture of
muriatic, phosphoric, and sulphuric acids; and if a similar trial
be made in the negative part, fixed alkaline matter is as quickly
developed.[65]
[65] Reflecting upon this and similar facts, it has occurred
to me that Voltaic electricity might be applied for removing
the blue colour in the skin, occasioned by the internal use of
nitrate of silver. I hope to be able very shortly to submit this
theory to the test.
Davy thinks that the acid and alkaline taste produced upon the
tongue during galvanic experiments, depends upon the decomposition
of the saline matter contained in the living animal substance, and
perhaps in the saliva; and he farther observes that, as acid and
alkaline substances are thus evidently capable of being separated
from their combinations in living systems by electrical powers,
there is reason to believe that, by converse methods, they might
also be introduced into the animal economy, or made to pass
through the animal organs; and the same thing may be supposed of
metallic oxides; and that these ideas ought to lead to some new
investigations in Medicine and Physiology.
He thinks it by no means improbable, that the electrical
decomposition of the neutral salts, in different cases, may
admit of economical applications; and that well-burnt charcoal
and plumbago, or charcoal and iron, might be made the exciting
powers for such a purpose. Such an arrangement, if erected upon a
scale sufficiently extensive, with the medium of a neutro-saline
solution, would, in his opinion, produce large quantities of acids
and alkalies with very little trouble or expense.
Alterations in chemical equilibrium are constantly taking place in
Nature, and he thinks it probable that the electric influence, in
its faculties of decomposition and transference, may considerably
interfere with the chemical changes occurring in different parts of
our system.
The electrical appearances which precede earthquakes and volcanic
eruptions, and which have been described by the greater number of
the observers of these awful events, admit also of easy explanation
on the principles that have been stated.
Besides the cases of sudden and violent change, he considers there
must be constant and tranquil alterations, of which electricity,
produced in the interior strata of the globe, is the active
cause: thus, where _pyritous_ strata and strata of _coal-blende_
occur,--where the pure metals or the sulphurets are found in
contact with each other, or with any conducting substances,--and
where different strata contain different saline menstrua, he thinks
electricity must be continually manifested; and it is probable that
many mineral formations have been materially influenced, or even
occasioned, by its agencies.
In an experiment which he performed of electrifying a mixed
solution of the muriates of iron, copper, tin, and cobalt,
contained in a positive vessel, all the four oxides passed along
the connecting asbestus into a positive vessel filled with
distilled water, while a yellow metallic crust formed on the wire,
and the oxides arranged themselves in a mixed state around the base
of it.
In another experiment, in which carbonate of copper was diffused
through water in a state of minute division, and a negative wire
was placed in a small perforated cube of zeolite in the water,
green crystals collected round the cube; the particles not being
capable of penetrating it.
By a multiplication of such instances, Davy remarks, that the
electrical power of transference may be easily conceived to apply
to the explanation of some of the principal and most mysterious
facts in geology;[66] and by imagining a scale of feeble powers,
it would be easy to account for the association of the insoluble
metallic and earthy compounds containing acids.
[66] During the contentions of the Neptunists and Plutonists,
alluded to in a former part of this work, specimens were produced
exhibiting the intermixture of mineral bodies, which was
completely hostile to all theory. These anomalies now receive a
plausible explanation from the agencies of Voltaic electricity.
"Natural electricity," observes our philosopher, "has hitherto been
little investigated, except in the case of its evident and powerful
concentration in the atmosphere. Its slow and silent operations in
every part of the surface will probably be found more immediately
and importantly connected with the order and economy of nature;
and investigations on this subject can hardly fail to enlighten
our philosophical systems of the earth, and may possibly place new
powers within our reach."
Thus concludes one of the most masterly and powerful productions
of scientific genius. I may perhaps have been considered prolix in
recording the progressive researches by which he arrived at his
results; but let it be remembered, that the great fame of Davy,
as an experimental philosopher, rests upon this single memoir;
and though the secondary results to be hereafter considered, may
be more dazzling to ordinary minds, yet in the judgment of every
scientific observer, they must appear far less glorious than the
discovery of the primitive laws. Let me ask whether Sir Isaac
Newton does not deserve greater fame for his invention of fluxions,
than for the calculations performed by the application of them? I
do not hesitate in comparing these great philosophers, since each
has enlightened us by discoveries alike effected by means invented
by himself. Not only did both unlock the caskets of Nature, but
they had the superior merit of planning and constructing the key.
I challenge those, who have carefully followed me through the
details of the preceding memoir, to show a single instance in
which accident, so mainly contributory to former discoveries in
electricity, had any share in conducting its author to truth.
Step by step did he, with philosophic caution and unwearied
perseverance, unfold all the particular phenomena and details of
his subject; his genius then took flight, and with an eagle's eye
caught the plan of the whole.--A new science has been thus created;
and so important and extensive are its applications, so boundless
and sublime its views, that we may fairly anticipate the fulfilment
of those prophetic words of Dr. Priestley, who, in the preface
to his History of Electricity,[67] exclaims--"Electricity seems
to be giving us an inlet into the internal structure of bodies,
on which all their sensible properties depend. By pursuing this
new light, therefore, the bounds of natural science may possibly
be extended beyond what we can now form any idea of. New worlds
may open to our view, and the glory of the great Sir Isaac Newton
himself, and all his contemporaries, be eclipsed by a new set of
philosophers, in quite a new field of speculation. Could that great
man revisit the earth, and view the experiments of the present race
of electricians, he would be no less amazed than Roger Bacon, or
Sir Francis, would have been at his." In our turn we may ask, what
would be the astonishment--what the delight of Dr. Priestley, could
he now witness the successful results of Voltaic research?--and
what would he say of that mighty genius who has demonstrated the
relations of electrical energy to the general laws of chemical
action?[68] It was his good fortune to have witnessed the discovery
which identified electricity with the lightning of the thunder
cloud: what would he have said of that which identified it with
the magnetism of the earth! Of this at least we may be certain,
that he would have expunged from his history the passage in which
he observes--"Electrical discoveries have been made so much by
accident, that it is more the powers of nature, than of human
genius, that excite our wonders with respect to them."
[67] The History and Present State of Electricity; by Jos.
Priestley, LL.D. F.R.S., &c. London, 1795.
[68] Dr. Priestley augured much from the talents of Davy. After
the publication of his first paper on Galvanism, he wrote to him
from America, and expressed the pleasure he felt on finding his
favourite subject in such able hands. Priestley died in 1804, and
therefore did not witness Davy's success.