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MARIOTTE AND VON GUERICKE

science


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



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 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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