DUFAY DISCOVERS VITREOUS AND RESINOUS ELECTRICITY
It has been shown in an earlier chapter how Von Guericke
discovered that light substances like feathers, after being
attracted to the sulphur-ball electric-machine, were repelled by
it until they touched some object. Von Guericke noted this, but
failed to explain it satisfactorily. Dufay, repeating Von
Guericke's experiments, found that if, while the excited tube or
sulphur ball is driving the repelled feather before it, the ball
be touched or rubbed anew, the feather comes to it again, and is
repelled alternately, as, the hand touches the ball, or is
withdrawn. From this he concluded that electrified bodies first
attract bodies not electrified, "charge" them with electricity,
and then repel them, the body so charged not being attracted
again until it has discharged its electricity by touching
something.
"On making the experiment related by Otto von Guericke," he says,
"which consists in making a ball of sulphur rendered electrical
to repel a down feather, I perceived that the same effects were
produced not only by the tube, but by all electric bodies
whatsoever, and I discovered that which accounts for a great part
of the irregularities and, if I may use the term, of the caprices
that seem to accompany most of the experiments on electricity.
This principle is that electric bodies attract all that are not
so, and repel them as soon as they are become electric by the
vicinity or contact of the electric body. Thus gold-leaf is first
attracted by the tube, and acquires an electricity by approaching
it, and of consequence is immediately repelled by it. Nor is it
reattracted while it retains its electric quality. But if while
it is thus sustained in the air it chance to light on some other
body, it straightway loses its electricity, and in consequence is
reattracted by the tube, which, after having given it a new
electricity, repels it a second time, which continues as long as
the tube keeps its electricity. Upon applying this principle to
the various experiments of electricity, one will be surprised at
the number of obscure and puzzling facts that it clears up. For
Mr. Hauksbee's famous experiment of the glass globe, in which
silk threads are put, is a necessary consequence of it. When
these threads are arranged in the form of rays by the electricity
of the sides of the globe, if the finger be put near the outside
of the globe the silk threads within fly from it, as is well
known, which happens only because the finger or any other body
applied near the glass globe is thereby rendered electrical, and
consequently repels the silk threads which are endowed with the
same quality. With a little reflection we may in the same manner
account for most of the other phenomena, and which seem
inexplicable without attending to this principle.
"Chance has thrown in my way another principle, more universal
and remarkable than the preceding one, and which throws a new
light on the subject of electricity. This principle is that there
are two distinct electricities, very different from each other,
one of which I call vitreous electricity and the other resinous
electricity. The first is that of glass, rock-crystal, precious
stones, hair of animals, wool, and many other bodies. The second
is that of amber, copal, gumsack, silk thread, paper, and a
number of other substances. The characteristic of these two
electricities is that a body of the vitreous electricity, for
example, repels all such as are of the same electricity, and on
the contrary attracts all those of the resinous electricity; so
that the tube, made electrical, will repel glass, crystal, hair
of animals, etc., when rendered electric, and will attract silk
thread, paper, etc., though rendered electrical likewise. Amber,
on the contrary, will attract electric glass and other substances
of the same class, and will repel gum-sack, copal, silk thread,
etc. Two silk ribbons rendered electrical will repel each other;
two woollen threads will do the like; but a woollen thread and a
silken thread will mutually attract each other. This principle
very naturally explains why the ends of threads of silk or wool
recede from each other, in the form of pencil or broom, when they
have acquired an electric quality. From this principle one may
with the same ease deduce the explanation of a great number of
other phenomena; and it is probable that this truth will lead us
to the further discovery of many other things.
"In order to know immediately to which of the two classes of
electrics belongs any body whatsoever, one need only render
electric a silk thread, which is known to be of the resinuous
electricity, and see whether that body, rendered electrical,
attracts or repels it. If it attracts it, it is certainly of the
kind of electricity which I call VITREOUS; if, on the contrary,
it repels it, it is of the same kind of electricity with the
silk--that is, of the RESINOUS. I have likewise observed that
communicated electricity retains the same properties; for if a
ball of ivory or wood be set on a glass stand, and this ball be
rendered electric by the tube, it will repel such substances as
the tube repels; but if it be rendered electric by applying a
cylinder of gum-sack near it, it will produce quite contrary
effects--namely, precisely the same as gum-sack would produce. In
order to succeed in these experiments, it is requisite that the
two bodies which are put near each other, to find out the nature
of their electricity, be rendered as electrical as possible, for
if one of them was not at all or but weakly electrical, it would
be attracted by the other, though it be of that sort that should
naturally be repelled by it. But the experiment will always
succeed perfectly well if both bodies are sufficiently
electrical."[1]
As we now know, Dufay was wrong in supposing that there were two
different kinds of electricity, vitreous and resinous. A little
later the matter was explained by calling one "positive"
electricity and the other "negative," and it was believed that
certain substances produced only the one kind peculiar to that
particular substance. We shall see presently, however, that some
twenty years later an English scientist dispelled this illusion
by producing both positive (or vitreous) and negative (or
resinous) electricity on the same tube of glass at the same time.
After the death of Dufay his work was continued by his
fellow-countryman Dr. Joseph Desaguliers, who was the first
experimenter to electrify running water, and who was probably the
first to suggest that clouds might be electrified bodies. But
about, this time--that is, just before the middle of the
eighteenth century--the field of greatest experimental activity
was transferred to Germany, although both England and France were
still active. The two German philosophers who accomplished most
at this time were Christian August Hansen and George Matthias
Bose, both professors in Leipsic. Both seem to have conceived the
idea, simultaneously and independently, of generating electricity
by revolving globes run by belt and wheel in much the same manner
as the apparatus of Hauksbee.
With such machines it was possible to generate a much greater
amount of electricity than Dufay had been able to do with the
rubbed tube, and so equipped, the two German professors were able
to generate electric sparks and jets of fire in a most startling
manner. Bose in particular had a love for the spectacular, which
he turned to account with his new electrical machine upon many
occasions. On one of these occasions he prepared an elaborate
dinner, to which a large number of distinguished guests were
invited. Before the arrival of the company, however, Bose
insulated the great banquet-table on cakes of pitch, and then
connected it with a huge electrical machine concealed in another
room. All being ready, and the guests in their places about to be
seated, Bose gave a secret signal for starting this machine,
when, to the astonishment of the party, flames of fire shot from
flowers, dishes, and viands, giving a most startling but
beautiful display.
To add still further to the astonishment of his guests, Bose then
presented a beautiful young lady, to whom each of the young men
of the party was introduced. In some mysterious manner she was
insulated and connected with the concealed electrical machine, so
that as each gallant touched her fingertips he received an
electric shock that "made him reel." Not content with this, the
host invited the young men to kiss the beautiful maid. But those
who were bold enough to attempt it received an electric shock
that nearly "knocked their teeth out," as the professor tells it.
|
