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 Burgundy. Mariotte demonstrated that but for the resistance
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 he deduced
the law that the density and elasticity of the atmosphere 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, 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 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." 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.