MARIOTTE AND VON GUERICKE
Working contemporaneously with Boyle, and a man whose name is
usually associated with his as the propounder of the law of
density of gases, was Edme Mariotte (died 1684), a native of
the atmosphere, all bodies, whether light or heavy, dense or
thin, would fall with equal rapidity, and he proved this by the
well-known "guinea-and-feather" experiment. Having exhausted the
air from a long glass tube in which a guinea piece and a feather
had been placed, he showed that in the vacuum thus formed they
fell with equal rapidity as often as the tube was reversed. From
his various experiments as to the pressure of the atmosphere 343n1313d he
deduced the law that the density and elasticity of the atmosphere 343n1313d
are precisely proportional to the compressing force (the law of
Boyle and Mariotte). He also ascertained that air existed in a
state of mechanical mixture with liquids, "existing between their
particles in a state of condensation." He made many other
experiments, especially on the collision of bodies, but his most
important work was upon the atmosphere.
But meanwhile another contemporary of Boyle and Mariotte was
interesting himself in the study of the atmosphere 343n1313d , and had made
a wonderful invention and a most striking demonstration. This was
Otto von Guericke (1602-1686), Burgomaster of Magdeburg, and
councillor to his "most serene and potent Highness" the elector
of that place. When not engrossed with the duties of public
office, he devoted his time to the study of the sciences,
particularly pneumatics and electricity, both then in their
infancy. The discoveries of Galileo, Pascal, and Torricelli
incited him to solve the problem of the creation of a vacuum--a
desideratum since before the days of Aristotle. His first
experiments were with a wooden pump and a barrel of water, but he
soon found that with such porous material as wood a vacuum could
not be created or maintained. He therefore made use of a globe of
copper, with pump and stop-cock; and with this he was able to
pump out air almost as easily as water. Thus, in 1650, the
air-pump was invented. Continuing his experiments upon vacuums
and atmospheric pressure with his newly discovered pump, he made
some startling discoveries as to the enormous pressure exerted by
the air.
It was not his intention, however, to demonstrate his newly
acquired knowledge by words or theories alone, nor by mere
laboratory experiments; but he chose instead an open field, to
which were invited Emperor Ferdinand III., and all the princes of
the Diet at Ratisbon. When they were assembled he produced two
hollow brass hemispheres about two feet in diameter, and placing
their exactly fitting surfaces together, proceeded to pump out
the air from their hollow interior, thus causing them to stick
together firmly in a most remarkable way, apparently without
anything holding them. This of itself was strange enough; but now
the worthy burgomaster produced teams of horses, and harnessing
them to either side of the hemispheres, attempted to pull the
adhering brasses apart. Five, ten, fifteen teams--thirty horses,
in all--were attached; but pull and tug as they would they could
not separate the firmly clasped hemispheres. The enormous
pressure of the atmosphere 343n1313d had been most strikingly demonstrated.
But it is one thing to demonstrate, another to convince; and many
of the good people of Magdeburg shook their heads over this
"devil's contrivance," and predicted that Heaven would punish the
Herr Burgomaster, as indeed it had once by striking his house
with lightning and injuring some of his infernal contrivances.
They predicted his future punishment, but they did not molest
him, for to his fellow-citizens, who talked and laughed, drank
and smoked with him, and knew him for the honest citizen that he
was, he did not seem bewitched at all. And so he lived and worked
and added other facts to science, and his brass hemispheres were
not destroyed by fanatical Inquisitors, but are still preserved
in the royal library at Berlin.
In his experiments with his air-pump he discovered many things
regarding the action of gases, among others, that animals cannot
live in a vacuum. He invented the anemoscope and the air-balance,
and being thus enabled to weight the air and note the changes
that preceded storms and calms, he was able still further to
dumfound his wondering fellow-Magde-burgers by more or less
accurate predictions about the weather.
Von Guericke did not accept Gilbert's theory that the earth was a
great magnet, but in his experiments along lines similar to those
pursued by Gilbert, he not only invented the first electrical
machine, but discovered electrical attraction and repulsion. The
electrical machine which he invented consisted of a sphere of
sulphur mounted on an iron axis to imitate the rotation of the
earth, and which, when rubbed, manifested electrical reactions.
When this globe was revolved and stroked with the dry hand it was
found that it attached to it "all sorts of little fragments, like
leaves of gold, silver, paper, etc." "Thus this globe," he says,
"when brought rather near drops of water causes them to swell and
puff up. It likewise attracts air, smoke, etc."[9] Before the
time of Guericke's demonstrations, Cabaeus had noted that chaff
leaped back from an "electric," but he did not interpret the
phenomenon as electrical repulsion. Von Guericke, however,
recognized it as such, and refers to it as what he calls
"expulsive virtue." "Even expulsive virtue is seen in this
globe," he says, "for it not only attracts, but also REPELS again
from itself little bodies of this sort, nor does it receive them
until they have touched something else." It will be observed from
this that he was very close to discovering the discharge of the
electrification of attracted bodies by contact with some other
object, after which they are reattracted by the electric.
He performed a most interesting experiment with his sulphur globe
and a feather, and in doing so came near anticipating Benjamin
Franklin in his discovery of the effects of pointed conductors in
drawing off the discharge. Having revolved and stroked his globe
until it repelled a bit of down, he removed the globe from its
rack and advancing it towards the now repellent down, drove it
before him about the room. In this chase he observed that the
down preferred to alight against "the points of any object
whatsoever." He noticed that should the down chance to be driven
within a few inches of a lighted candle, its attitude towards the
globe suddenly changed, and instead of running away from it, it
now "flew to it for protection" --the charge on the down having
been dissipated by the hot air. He also noted that if one face of
a feather had been first attracted and then repelled by the
sulphur ball, that the surface so affected was always turned
towards the globe; so that if the positions of the two were
reversed, the sides of the feather reversed also.
Still another important discovery, that of electrical conduction,
was made by Von Guericke. Until his discovery no one had observed
the transference of electricity from one body to another,
although Gilbert had some time before noted that a rod rendered
magnetic at one end became so at the other. Von Guericke's
experiments were made upon a linen thread with his sulphur globe,
which, he says, "having been previously excited by rubbing, can
exercise likewise its virtue through a linen thread an ell or
more long, and there attract something." But this discovery, and
his equally important one that the sulphur ball becomes luminous
when rubbed, were practically forgotten until again brought to
notice by the discoveries of Francis Hauksbee and Stephen Gray
early in the eighteenth century. From this we may gather that Von
Guericke himself did not realize the import of his discoveries,
for otherwise he would certainly have carried his investigations
still further. But as it was he turned his attention to other
fields of research.
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