VIII(B)
Second letter to
Martin Van Marum
Sir,
Another course
[at Pavia University] has delayed by some weeks my
continuing the exposition of my experiments on
animal electricity. Since I do not precisely
remember the point reached, I start here a little
behind: this small
loss of time and paper is a slight
inconvenience, but in doing that, I am paying for
my loss of attention.
Let us now
come to the experiments I performed on my tongue(1).
When,
instead of using silver or tin blades, we use
copper or iron armatures or other metals not
really suitable for this experiment, the taste
felt on the tip of the tongue will not be stronger
than the one produced when using an ordinary [electrostatic]
Machine run at a modest speed, and it could be
even null or very weak, particularly the alkaline
taste which is much less easily perceived.
It
is then the electric fluid that produces the
sensation on the tongue, even when we do not use
artificial or Machine electricity, i.e. when we
apply to the tongue the two metallic blades of tin
and silver and establish communication between
them. The cause is the same i.e. the same movement
of electric fluid, since the sensations are the
same as those produced by the electric current
generated by the ordinary Machine, with only minor
occasional differences. We must also notice that
all this is caused by a considerable amount of
fluid that flows quietly from one part to the
other. Could we believe that the simple
application of two different metals would produce
such circulation of electric fluid? How can
this happen? I will say something about this later.
Let me draw attention to what I just said with the
term different
metals: there is the whole secret.
In fact, two
metals of the same type do not produce the
sensations described above, at most a sensation of
a small intensity; so no taste on the tongue, no
convulsion in any muscle, if we apply two pieces
of the same metal, having equal mettle and surface.
I can state that, having tried almost all the
combinations of different metals, almost all have
produced more or less the effect (and sometimes
two metals of the same kind having only some
difference in the alloy, the mettle, the hardness,
roughness or cleanliness of the surfaces). There
are nevertheless metals that have produced only
very small effect, like gold and silver, copper
and iron, tin and lead. In general I believe that
I can separate metals into three classes, high,
middle, and lower, assigning to the first silver,
gold and platinum; to the second copper and
several of its alloys, iron and some semi-metals;
to the last zinc, lead and tin. The effect is
largest, i.e. the contractions of the muscle more
violent, and the taste on the tongue stronger,
when we employ for our blades a metal of the first
class, and for the other a metal of the third one,
particularly silver and tin; and on the contrary
the smallest when we oppose two metals of the same
class, like gold and silver, tin and lead, copper
and iron, finally we obtain a mediocre effect if
we combine a metal of the middle class with
another of the lower or of the high class, though
in the last case, e.g. with copper or iron on one
side, and silver or gold on the other, the effect
is less than mediocre. This proves that the metals
that I have put in the middle class, the iron,
copper and so on, appear much closer to the high
than to the lower class.
I confess that
it is difficult to understand why two metals of
different classes applied in two external parts of
the animal, even on two similar muscles, or on two
points of same muscle, disturb the quietness of
the electric fluid and cause it to pass
continuously from one part to another. This can
happen only in one of two ways: either one of the
metals tends to give electric fluid, while the
other tends to take it in the places that are in
contact [with the metals], or both parts tend to
take [electric fluid] from the parts in contact,
but with different forces; or finally that they
tend with different force to give [electric fluid]
to the respective parts, as I am inclined to
believe. For each of these modes we have a
transport of electric fluid from one part to the
other, every time the contact metals communicate
between each other, and a continuous current is
generated with a certain rapidity, if the body to
which these metals are applied being a good
conductor (as in our case are the nerves and the
living flesh full of fluids, as in the tongue) the
electric fluid,
when it accumulates, can easily flow from
one part to another.
But, will we be able to understand where
this tendency of the metals originates, i.e. this
force of attracting or giving the electric fluid
to other conducting bodies, due to the fact that
each of them has its own natural amount, where it
is in equilibrium? How is this equilibrium
disturbed by mere contact, without any brushing? I
am saying without any brushing; this must be
considered surprising, because the efficacy [of
the brushing] in breaking such equilibrium and
obtaining the passage of electric fluid from one
body to another has been known for a long time.
The fact is known but the reason is not, and this
should be worth studying more than it has been up
to now. But it is precisely because the fact is
too well-known that it has not been well studied.
We
know only the effect, i.e. the addition or piling
up of electric fluid over one of the brushing
surfaces at the expense of the other one (in this
consists the electrisation of surfaces by brushing)
and various circumstances modifying this effect.
Among others, it depends particularly on
the nature and quality of the material used to
brush one or other material. The effect depends
also on the surface that can be rough or flat and
polished, on the duration of the brushing, on the
temperature and other circumstances all of which
can change the situation from white to black; so
that there is nothing so difficult, sometimes, to
determine which one of the two brushing materials
will acquire or lose electric fluid. But it is
true in general that, more than the above
accidental circumstances, it is the nature and
quality of the materials involved that determine
the effect. So, past experience had already taught
Physicists that metals differ considerably among
themselves in relation to the effect, some of them
being more apt to give than to receive, so that a
silk ribbon, a resin
plate,
a piece of wood dried in the oven, a sheet of
paper, etc., brushed with a blade of gold or
silver electrify in plus, brushed with a blade of tin or lead electrify in minus.
Once
established, this different attitude of the metals,
makes it less difficult to understand that, even
without brushing, a similar breaking of the
equilibrium and the accumulation of the electric
fluid in one material at the expense of another
can be obtained through simple contact and the
application of said metals to imperfect conductors.
Even the brushing itself does not cause the
transport of the fluid, but rather it creates more
precise contact and causes a multiplication of the
particles from one material with the ones of the
other material. We can say that what happens to
the electric fluid derives only from the contact
between the surfaces, due to some change caused by
the contact of the mutually attracting forces
between the particles and the fluid, or to its
elasticity, etc. What is certain is that there is
no real need of brushing and that a blow or any
kind of pressure is enough. Indeed, a few years
ago I already succeeded in electrifying glass and
resin plates perfectly clean and dry, by
depositing them carefully in a bath of mercury, or
on cushions covered with metal leaves, equally
clean and dry, and taking them out just as
carefully. Well then, today's experiments tell me
that there is no need for even slight pressure in
order to obtain that metallic blades applied to
nerves or muscles obtain a similar transport of
electric fluid, since as long as this application
lasts, the fluid continues to be pushed as it
would do with continued brushing. It must not be
believed that this happens because the two metals
have been put and stay in contact with anomalous
parts, since further experiments, too long to be
reported here, show that the same results are
obtained if they are put in contact with other
kinds of material, such as water or moistened
paper, etc.
We can then
conclude that only the contact of metals with
sufficiently good conducting materials, though far
less good than metals, materials called non-electric,
obtains on these what was believed to be
obtainable only through friction or percussion on
non-conductors or idioelectrics. I moreover
believe that precisely for those non-conductors,
which the fluid has difficulty entering as well
leaving, friction is required as being more
efficient, while it is not so for sufficiently
good conductors where, the electric fluid being
able to move much more easily, only the presence
of, or mere contact with metals having their own
force for releasing or attracting the electric
fluid is sufficient to produce the effect. This is
an important discovery, since, if it does not give
a more satisfactory reason for the breaking of
equilibrium and for the transport of the electric
fluid caused by friction itself, it
tends to generalize such effect and to
correct our ideas on the subject.
But, assuming
that simple contact is, in the circumstances described,
equivalent to a certain extent to friction, as we
have just explained, we can raise another serious
difficulty that seems to break any analogy: friction
on gold and silver is more effective in giving
electric fluid to the bodies on which friction is
applied than tin and lead. While when we simply
apply strips of the same metals to animal parts or
to bodies soaked with water, etc, it is just the
opposite (as is proved by my experiments on the
tongue, just described), because the electric
fluid flows from the tin or lead in the parts they
touch and immediately
flows to the parts in contact with the gold or
silver and enters these. I really do not know how
to answer but I can note that, as the amount of
brushing sometimes
changes the results, analogously bodies that would
give [electric fluid] with a small pressure during
brushing would receive with strong pressure, or
vice versa. A similar contradiction exists between
the brushing or percussion of those metals and
their simple contact with no pressure or brushing.
Leaving these
explanations that would be too long, let us stay
on the fact. The electric fluid flows
spontaneously, in our experiments, from the tin or
lead strip to the parts of the animal touched by
the strips, penetrates into those parts down to a
certain depth and flows toward those in contact
with the gold or silver blade, flows in this
toward the tin blade that deposits it again softly.
In this way continuous circulation is established,
as long as the communications hold. Now, if the
current of the electric fluid, which penetrates,
as I just said, to a certain depth in the animal's
fibers, the depth being not great but sufficient
only to determine an easier road from a part
touched by one blade to the other one, if, I say, on
its way this current encounters some nerves,
they will be irritated (as we have learned
previously) and these irritated nerves will excite
either sensations or muscular movements, depending
on their function.
I say
sensations or muscular movements according to the
function proper to the nerve, because my
experiments on the tongue prove that a muscle
seems susceptible of all contractions and
voluntary movements, and the electric fluid can,
while penetrating, stimulate very sensitive nerves,
without nevertheless causing any movement if, by
chance, the role of these nerves is different,
i.e. if they are nerves assigned only to
sensations. Now such are those belonging to the
tongue, particularly on its tip. This explains why
the only effect caused by the electric fluid is a
sensation of taste, with no convulsions at all. In
accord with this idea, I thought that the nerves
for the voluntary movements of the tongue are
implanted in its root. This could be proved by
tearing out the tongue from an animal and applying
a metallic blade to its root close to the
insertion of such nerves. We could obtain sharp
movements which a tongue is capable of. I have
done such an experiment with success, as I had
predicted. Since this experiment is one of the
most beautiful and instructive, I am going to
describe it in some detail.
I
cut the
tongue from a newly slaughtered lamb, close to its
root, and applied tinfoil close to the cut and a
silver spoon to one of the faces, then
establishing communication as is necessary between
the two metal armatures, I had the pleasure of
seeing the whole tongue tremble sharply, in
particular raising its tip or bending from one
side to the other with spasms. And this every time
I made the two metals communicate. I have repeated
this experiment on a calf’s tongue, placing the
tinfoil on its root and then I put the tongue on a
silver plate as the second metal part. It worked
with equal success. I then repeated the same
experiment with the tongue of other small animals,
such as rats, chicken and rabbits, obtaining the
effect almost
every time. I say “almost”, since
sometimes it did not work on the tongue of small
animals, or because the tinfoil could not be
applied conveniently and in the right place, or
because the nerves for the tongue movement could
not be reached, or because the tongue was already
cold and had lost its vitality which does not last
for long in the muscles of warm-blood animals,
particularly in the tongue.
There is
therefore a very marked distinction between the nerves
that can be called movement
nerves and the sensation
nerves; and the experiments just described on
the tongue obviously show if the nerves that the
electric fluid puts in motion by the contact of
two different metals are irritated, what their
function is,
and what the resulting effects are,
i.e. movement and sensation. It must therefore be
concluded as a legitimate consequence that the
action of such an electric current is not to
irritate the muscle to the point of exiting
immediate contractions, but to give the nerves the
possibility of putting their
role into action, as I have proved with
other arguments. Indeed, if the muscular fibers
are excited, why would this not produce movements
of the tongue, which is so mobile, when it
penetrates the tip and flows through more than
half of it except its root? In such a case, it
generates only a sensation of taste, in accord
with the nature of the nerves involved in this
case, which are the nerves of taste.
Coming back to
the circulation of electric fluid described above,
I have shown how it flows from the tin blade in
contact with animal parts or other
conductors and penetrates more or less within them,
reaches the other parts covered by the silver tag,
where it enters via the conducting arc, and then
returns to the tin strip, continuing the tour. I
do not believe it useless to recall the necessity
of two different metals for
that to happen. If the two armatures are
made of the same metal, with the same hardness,
the same mettle and the same polishing, there is
no reason why one of the strips should give
electric fluid more easily than
the other to the animal part in contact. At
most each tag would give or take a small amount;
but, the forces being
balanced, we could never have current or
circulation. That can happen only when the
respective metals are different; particularly due
to the qualities of the metals, one blade is more
prone to give or to receive than the other one. [It
can also be noticed that] the natural tendency of
one part is not to receive while the natural
tendency is to give, since in all cases it can be
understood that the electric fluid pushed in one
direction according to the prevailing force, must
circulate continuously, provided that it is not
broken by an interruption of the good conductors.
Another
point I would like to recall is that the amount of
electric fluid set in motion by the two metals is
not small, as one might imagine. On the contrary
it is very considerable to judge by its effect on
the tongue and the amount of the same fluid that
one is obliged to produce on the same organ using
the Electric Machine in order to obtain an equal
effect, i.e. in order to get the same acid
or alkaline taste, of the same intensity,
as I already pointed out. It is certainly true
that in both cases the fluid current, though
abundant, is so slow and exhibits such little
force and tension that no effect can be seen on
the electrometer and that it can be easily stopped
by bad conductors: in a word it is a rich current,
but sweet and pleasant. It should therefore
not be slowed further, by avoiding interposed bad
conductors (such as wood, fabric, leather, even
paper of little thickness, and a dry
or lightly moistened membrane, etc), if we
want to tickle the nerves to the point of exciting
either the taste on the tongue or contraction in
the other muscles.
As I have
already said, the electric current is moved and
kept moving by the different tendencies of
differing metals to push the electric fluid or to
suck it, when the metals are simply applied to the
surface of a body that conducts well enough but is
inferior(*)
in that sense to materials that are classified as
non-conductors (such different tendency reveals
itself also when the metals are rubbing bodies of
the insulator or idio-electric class).
I also said that the phenomenon here discussed is
not exclusively found using organs or substances
from living animals, but also using other bodies
having the property as indicated of being good
conductors. I have formally declared this by
indicating as example a piece of fabric or wet
card. Am I required to adduce experiments? Here
are some demonstrative cases. I take a glass of
water and put in the water a strip of tin, folded
over the edge of the glass in order to keep it in
place (or, in case of a thin sheet, i.e.what is
called “silver paper”, I just place it on the
surface of the water). Then I put the tip of my
tongue in the water, close to the tin strip (but
not in contact with it). Then I apply to the back
of my tongue the convex part of a silver spoon and
move its handle so that it comes into contact with
the tin strip folded over the glass rim.
Immediately my tongue feels the acid taste, and
this as long as the contact lasts. It is clear
then that the electric fluid flows from the tin
strip to the water. It crosses the water and
arrives at the tip
of the tongue, penetrates into it and reaches the
part that is in contact with the silver spoon. It
goes through its handle and arrives at the tin
strip. It is almost useless to remark that if the
experiment is done by first inverting the metals,
with the silver tag immersed in the water and the
tin one
applied under the tip of the tongue immersed in
the same water, it would not feel anything if the
two metals were not well chosen for this sort of
experiment. Even here it is much easier to excite
the acid taste than the alkaline one.
Proceeding in
another way, I immersed in a pitcher of water a
tin strip and a silver one separated from the
first, each with one of its extremes bent over the
edge of the vase. Two silver spatulas or spoons
are applied to the tongue, one over its tip, the
other on the upper or lower surface. The first is
then put in contact with the tin strip, the second
with the silver one. While only one contact is
made, no sensation is felt, but as soon as both
contacts are made the tongue feels the acid
sensation. This is certainly not the effect of the
metals applied to the tongue, since both armatures
are of silver and therefore cannot produce any
effect. The push to the electric fluid that
determines its circulation, in this experiment,
comes from the other strips immersed in the water,
i.e. it flows from the tin strip into the water,
reaches the silver tag and then goes to the silver
spoon applied to the tip of the tongue.
I have also
done experiments in which the tin strip and the
silver one were applied to a piece of fabric card,
both moistened, obtaining a successful result,
only the sensations were weaker as those materials
were less soaked or non-existent if they were not
soaking but only moist.
Just another
word about the taste excited on the tongue by the
contact of two metals.
The
easiest way to feel this taste is to apply a small
tape of tin foil across a silver tag, e.g. a coin,
and to apply the tip of the tongue to the border
between the two metals. It is a surprise not to
feel anything when the tip of the tongue touches
only the silver or the tin.
When
the tip of the tongue touches both metals, very
strong acidity (since of the two tastes the acid
wins in this experiment) is felt, just at
the border between the two metals,. Now, should not we recall the similar reason why we feel a
piquant taste found in water and that which
enhances the taste of beer when we drink from a
metal cup, and the latter particularly from a tin
mug? What
is certain is that the metals set the
electric fluid in motion, not only when
applied to animal parts but also to other
sufficiently good conductors, water, soaking
materials, etc., as I have just shown. Now, it is
more than probable that two metals of different
species are needed for establishing a current of
the fluid large enough to excite a strong taste on
the tongue. A single metal could already do
something, like disturbing the electric fluid and
exciting the tongue a
little. Hence, I believe, the slight more
or less perceivable taste
of each metal simply touched with the tip of the
tongue, which is called a metallic taste. It is
basically the same experiment with two metals, but
infinitely more imperfect. Perfection is
approached when the tongue touches metal and water
(or some other beverage) drinking from a metallic
vase, and much more when doing the experiment
described above, putting a metal tag in a glass of
water, plunging the
tip of the tongue into the water and
touching the flat of the tongue and the immersed
metal plate with another strip of different metal.
I have more to
tell you, Sir, in a third letter(2)
that will be not so long. What remains to be said
is nevertheless the most important. We must decide
whether, after all the experiments that do not
prove the existence of real animal electricity
derived from vital forces but simply artificial
electricity excited by a means unknown before,
whether, I say, animal electricity exists (as I
believe) and on what experiments it can be proved.
I am, Sir,
with all friendliness,
Yours
A.Volta
Como,
11 October 1792
[Translated
from French by Luigi Dadda, Politecnico di Milano,
October 2000]
[Revised
and completed by John Coggan, Oxford University]
(*)
Translator’s note: Volta is
referring to its position in his classification
