xv
LETTER
TO
ORAZIO DELFICO
Pavia, 13 April 1795
My Dear Friend and Esteemed Patron,
I begin with an apology to Your
Lordship for the long delay in answering Your two letters, the first of which I
received at the beginning of autumn and the other at the beginning of last
winter. I had the desire to answer you after I had talked of your commission to
the King’s Ambassador. I reached Pavia in the middle of November, and I talked
to him but I could not manage to get him to execute comparable Electrometers
this winter. They require long work and before our summer holidays he has no
time left to carry it out. He promises to do so before I leave Pavia, since he
needs my assistance, which I will offer with great pleasure.
I have been proceeding with my studies
into the effect of electrical semiconductors, especially the metallic ones, on
nerves and muscles; I mean those metal Conductors put in contact with each other,
or alternated with conductors of another class, i.e. water or other non-oily
fluids, or even solid bodies containing a sufficient quantity of such fluids;
all of which, both fluids and liquids, I refer to by the name of moist
conductors, dry conductors being those of the first class, that is
all metals and semi-metals, various mines and pyrites (many of them, especially
pyrites, were no less good conductors than pure, perfect metals), and animal and
vegetable carbon. All my experiments and observations persuade me that the
movement of the Electric fluid is given by the conductors themselves, applied
externally, by their own property, which acts from simple contact between two of
them different from each other, there being no need to imagine an imbalance of
the fluid in the animal organs, e.g. between muscle and nerve, or between the
internal and external parts of a muscle, as GALVANI's followers assert.
The situation is this:
conductors, singularly metallic, cannot conduct electricity already excited or
unbalanced, as so far believed, since they have the virtue and capacity of
exciting it, of interrupting the quiescence of the electric fluid, of exciting
and put it in movement, every time it finds itself in a completed circuit, or
when a conductor of the class of moist conductors finds itself between two, I
say, of these dissimilar, or substantially as gold, silver and mercury on
the one side, and lead, tin and zinc on the other, or even only for an
accidental difference in temper or smoothness, etc... Or a dry conductor between
two different ones, like pure water, brine, vinegar, spirits of wine, ink,
liquors, acids, alkalis, etc… milk, serum, blood, mucus, saliva, urine, bile,
etc..
With this principle, demonstrated by direct experience, all phenomena of the
supposed, for me groundless, animal Electricity, are explained. It also
explains how sometimes under the most favourable circumstances the contractions
of the frog prepared in GALVANI's manner, without any metals, are excited by
simply by putting its legs directly in touch, or particularly the gastrocnemius
muscle - or better touching its smooth and whitish part to the base of the
spine, that is to the back muscles oozing blood or other humour, or sciatic
nerves similarly moistened. With this and other similar experiments Galvanians
presume to triumph and to prove a real imbalance in the Electric fluid existing
between leg muscles and nerves, or a real electric charge excited by
organic virtue. However, I am rather coming to believe, I am even persuaded,
that organs here are not passive, that is merely Electrometers,
and that the movement transmitted to the fluid, the original impulse, derives
from the contact of two conductors somehow different, as they are on the surface
and where the gastrocnemius muscle, on the one hand, with its tendon
covered by its bright-white membrane is brought in touch, on the other hand,
with the crural nerves bleeding or moistened with some other humour, or the
bare, equally wet flesh of the frog's back freshly prepared. I am even more
convinced of this theory, since convulsions are obtained which are so difficult
to achieve and only seldom observed without any metal, and only in very lively
frogs, freshly prepared. There is no effect by putting in contact any part of a
leg with any part of the back, but the effect is obtained by bringing into
contact certain specific parts or even points, (which ones depends on the
circumstances) but most of all the afore-mentioned tendinous white parts towards
the foot, with those red and leaking humours on the back, near to the cut, since
when by chance the effect is difficult to obtain, or it is not obtained after
some time, it is often re-obtained by wetting one of the extremes, singularly
the back, with saliva, brine, spirits of wine, ink and even blood, and then
trying to obtain contact between the moistened points The contact does not
produce anything, at the beginning, nor after wetting the parts and washing with
pure water. All these experiments – and many others carried out in different
ways, which would be too long to present here – clearly show how Electricity
acts, and that Electric fluid is excited and set in motion only by reason
and in consequence of the dissimilarity between the two conductors which are put
into contact. And if the effect with all the dissimilarity of the conductors of
the humid type is very small in comparison with that obtained with two
dissimilar ones of the dry class, such as silver, tin or zinc, the reason
is that not only are the latter undoubtedly better conductors than the former,
but they are also more excellent motors. In other words, they have the
afore-mentioned virtue which I discovered, of bringing quiescence to an end and
setting the electric fluid in motion at a higher level.
After all, the principle is
the same and extends to all conductors, as I supposed from the beginning (which
conjectures I shared, as early as summer 1792, with some of my correspondents
and especially Mr Van Marum in two letters that have, I think, been published).
As this idea spreads, it acquires more and more importance.
Both
the capacity of the conductors, especially metallic, of exciting the electric
fluid and setting it moving – since they are different, the one pushing and
the other pulling, or one prevailing on the other in pushing or pulling – and
the great excitability of the nerves that, being in the conducting circuit, are
assailed by the current, especially when this is compelled to pass through the
nerves, justify the sharp taste excited on the tongue, the lightning in the eye
and the feeling of pain inside the eyelids towards the lachrymal glands, as well
as open wounds or recent sores, any time these sensitive parts enter the
conducting circle, part of which consists of two very dissimilar metals,
especially silver and zinc in direct contact – or with interposition of other
metals and the remaining part of the deferent circuit consisting of humid
conductors. These sensations are a direct effect of the electric impulse on
respective nerves. But when those flooded and crossed through by the electric
fluid in its course are not the sense nerves but the motion nerves, which govern
flexor and extensor muscles, and the corresponding motion is excited in those
muscles, such motion can be and I believe is entirely or mostly an intermediate
or secondary effect, certainly not a direct one. In other words, the electric
fluid stimulates and excites the nerve, which in turn excites the muscle to
contraction; we do not yet know how. In fact there is no need for the fluid to
flood through the nerve all the way to the muscle, as asserted by the supporters
of GALVANI’s theory. No, there is no need for the electric current to reach
the muscles itself; it is enough that it crosses through a short stretch of the
nerve only; it is enough that it passes through only the thickness of the
intersection of the nerve with the muscle. If a turtle's or a big frog's bare
crural nerve is slightly pressed with silver tweezers, an inch and a half above
where the nerve joins the muscles – and one or two lines further up pressed
with tin or better zinc tweezers; when the two tweezers are put in contact
directly or by means of a third metal, violent convulsions of the muscles will
begin. I usually excite convulsions also by pressing the nerve with only one
pair of tweezers, with one leg of silver and the other of zinc.
These
experiments of exciting contraction of the muscles by letting the electric fluid
flow through a short section of nerves only, by pricking the nerves themselves
and not the muscles, succeed with the nerves of voluntary movements, not with
those of necessary and involuntary movements. Stimulating thus the heart or
ventricular nerves, neither heart nor ventricle enter into contraction. To
excite the heart to double its beats, or to arouse those which are feeble or
have already ceased, it is necessary that the current of electric fluid spread
and cross through the very substance of the heart itself. And still by this,
even with the living spark from the electric machine the heart is excited very
little and with difficulty. This muscle, like the other involuntary ones, is
much more susceptible to chemical and mechanical stimulus, than to electric
impulse and even requires direct application of the impulse itself. On the
contrary, the muscles of voluntary movements (1)
feel electric impulse very easily
and do not need direct application of it or the current of electric fluid to
reach them. It is enough that the fluid pass through a very short stretch of the
nerves governing such muscles, as demonstrated by my experiments reported above
and by other research.
All this and other analogies have brought me to the idea that
electric fluid is used by the will to produce contraction in the muscles
submitted to it. And it is true that heart and ventricle or intestine muscles
being more appropriate to chemical and mechanical impulse, Nature itself makes
use of such impulses to excite contractions, that is using blood first of all
and the gastric juices, food and air. Why should we not think that Nature itself
uses the electric agent for those nerves and muscles which are more appropriate
to this particular impulse? And all the more so that it does not need any
further effort to produce or elaborate such a fluid, it does not need either
secretor organ to split it or vases to contain it, since it can be easily found
in any body, ready to react to the slightest impulse in conductors such as the
body's humours, juicy nerves, and all humid fibres. It does not need a great
effort, since even the slightest one is enough, as seen when an electric current
is excited invading and crossing a small stretch of nerve, and thus causing
evident contractions of the muscle. This current is not strong or particularly
vibrating, but extremely mild, similar to that caused by the simple contact of
two dissimilar metals It is a current, whose tension or effort does not cause
the most mobile electrometer to vibrate, nor to gain the slightest impediment or
resistance. In this way, there in the brain, where all nerves end, the action of
will would obtain the movements required by muscles and organs, simply by
applying a very slight movement to the electric fluid, to determine a mild
current in nervous branches and threads. It is not necessary to apply this
movement up to the point where the nerves enter the muscles which they control,
but only over a more ore less short length of nerve, a length that might not
exceed, or not by much, the centre of the brain itself. – Quam parvo
molimine res magna!
Our theory is enhanced when we consider that the structure of nerves is not
like a vacuum tube or a pipe, through which any liquid fluid can pass with the
quickness necessary. But if no spirituous or ethereal liquor can be movable or
penetrating enough to leap across those nervous paths, and the electric steam of
the junction has the particular faculty of flowing with ease through them as
through any other deferent conduit, which other fluid could perform in voluntary
movements the functions already ascribed by Physiologists to a supposed nervous
fluid, vaguely defined as animal spirits?
This fluid or animal agent
hitherto never defined or understood, is now coming to be specifically known,
when we say it is the real common electric fluid, many properties and laws of
which, if not its innermost nature, experimental Physics has discovered and made
clearer.
Thinking thus neither Physics nor Medicine will regret my overturning of a
system of animal electricity (that of GALVANI and his followers), which I have
replaced with another, where our electric fluid makes an appearance in the
animal Economy, and also plays an important role in the noblest part, to whit
that of voluntary movements, being in a sense a function of the will itself.
Other Physiologists had already supposed an analogy between animal spirits and
aether, light, or electric fluid. Some of them were even persuaded of a perfect
identity between the electric fluid and these spirits, but they were mere
hypotheses, without proofs and probable arguments, on the basis of then current
knowledge of electric phenomena and laws, and in accordance with the role the
electric fluid was supposed to play, which did not conform to, and was indeed
totally unlike the laws themselves.
However, this belief was opposed by HALLER and rejected by most people, with
many medical and physical arguments. Now the new point of view of my hypothesis
is not subject to those many objections, and the hypothesis itself is not vague
nor arbitrary but based on defined correspondences and analogies.
I had many other things to say, but the letter is already too long.
So I will finish by begging you to continue granting me your patronage and
friendship, and to pass my greetings on to your most worthy uncle.
I obsequiously remain your
most faithful and obedient servant and friend,
ALESSANDRO VOLTA
