THE AIR-PUMP
WHILE Galileo’s plan of work was to consider a theory, and
then to test it by experience, other men of science went to work in the
opposite way.
[Illustration: Fig. 36.--Shape of tube used by Boyle to show that air
exerts pressure.]
They made a large number of experiments, and gradually allowed the
results of the experiments to force them into a certain way of looking
at things. One man of science who adopted the second method was Robert
Boyle, an Irishman, who made himself famous by his studies on what he
called “the spring of the air,” meaning the pressure which compressed
air can exert. He took a long glass tube and bent it into a hook at the
lower end, which was closed. He then poured in mercury through the top,
until the bent end contained only a little air, which was compressed
by the weight of the mercury in the long tube. He poured in more and
more mercury, and found that the space filled up by the air became
smaller, the greater the pressure of the mercury was. He found, in
fact, that the air behaves very much like an elastic spring, which can
be stretched and compressed.
After some experience of this kind, Boyle heard of an apparatus which a
German had invented, and with which he performed the most astonishing
experiments. This man’s name was Guericke. Guericke had the idea of
finding out all about the stars, and how they moved through space. He
believed that they did not move through air, because if they did they
would gradually be stopped by the resistance of the air. He decided
that if he wished to find out how they moved, he would have to obtain a
space similar to that in which the heavenly bodies moved. This space,
he said, would have to be an empty space. He therefore tried to make an
empty space.
The first method Guericke adopted was a very curious one. He took a
big cask filled with water, and containing only one opening. Into that
opening he screwed a pump, and tried with all his might to pump the
water out of the cask. This was more difficult than he had expected,
for the air, pressing on the water with the force of fifteen pounds per
square inch, held the water in the cask. If the opening had been free,
the air would have got into the cask in bubbles, and would have pressed
the water out through the opening. But Guericke did not allow any air
bubbles to get into the cask, and so he had to work against the whole
pressure of the air in trying to pump the water out of the cask. If he
could but pump it out, he was convinced that he would leave only empty
space in the cask.
[Illustration: Fig. 37.--Guericke’s experiment of trying to draw Water
out of a full cask in order to leave a Vacuum.]
Well, he did get a large portion of the water out of the cask by
working the pump with all his might, but as soon as the water flowed
out, he heard a gurgling, or bubbling, sound made by the air which was
pressed in through all the joints which were not quite air-tight. Then
he put one cask inside another, but he did not succeed much better,
because again there was a noise resembling hissing and the twittering
of birds. This experiment continued for several days, and in the end
the cask was found full of air. Guericke therefore was convinced that
he could not get an empty space while using a wooden cask.
He then tried a ball made of beaten copper. Three men pumped the air
out of this ball, and no sooner had most of the air been pumped out
than the ball collapsed with a loud report, crushed by the pressure of
the outside air. He then had another copper globe made. It was divided
into halves, and these were joined by means of a ring of leather which
was well soaked in a mixture of wax and turpentine. When that globe was
emptied of air by means of the pump, the halves stuck so well together,
that no human power was able to separate them; but when air was let in
by turning a cock, they immediately fell apart. This experiment was
performed again and again, and on one famous occasion it was performed
before the German Emperor and his Parliament. On this occasion the two
half-globes were fixed so firmly together by the pressure of the air
merely, that it took sixteen horses to pull them apart. The total force
with which they stuck together was about equal to the weight of a ton.
When Boyle heard of these experiments, he made an air-pump of his own.
He arranged it so that he could easily pump the air out of a glass
globe which had a valve.
[Illustration: Fig. 38.--Old Print showing horses trying to pull apart
two Magdeburg Hemispheres from which the air had been pumped.]
He could put in various objects from above by opening the valve, and
then pump out the air and see what happened. The whole globe could
be taken off the pump very easily, since it had a brass nozzle which
fitted into the pump. The brass nozzle was fixed to the glass by means
of a cement consisting of pitch, resin, and potash.
After the air-pump had been discovered, everybody set to work to try it
in various ways. Boyle brought a small closed bladder into the globe
and worked the pump. He found that the bladder at once expanded, and
finally burst; evidently because the pressure of the air inside the
bladder was no longer balanced by the pressure of the air in the globe.
He also put a little cup of water under the air-pump, and found that it
gave off bubbles and finally appeared to boil, although it was not hot.
Boyle thought that water could be converted into air, but in this he
was wrong, because we know now that steam is given off from water under
low pressures of air even when the temperature is very low. Boyle found
also that water falling in a vacuum makes the sound of some metallic
object.
Meanwhile, Guericke was also actively improving his apparatus, and
soon he was able to pump out nearly all the air, and to leave only
one-thirtieth of the original amount in the vessel he employed. He
next succeeded in proving that a vacuum does not allow sound to pass
through it. He placed a clock into the vacuum, and as he pumped the air
out more and more, the striking of the clock was heard more and more
faintly, until it became quite inaudible. He was so convinced of the
powerful effect of the suction of an empty space, that he said that if
a man were to blow his breath into a large vacuum, that breath would
be his last.
[Illustration: Fig. 39.--Guericke’s experiment to show the pressure
upon the piston of a cylinder from which air was being extracted.]
The Marshal of the Emperor refused to believe this, but Guericke
undertook to convince him of the truth in another manner. He
constructed a cylinder with a piston which moved in it air-tight. This
piston was suspended by a rope over a pulley, and to the end of the
rope twenty small ropes were attached, and each of these was held by a
strong man. They pulled up the piston until it was at the top of the
cylinder. Into the bottom of the cylinder a small hole was bored, and
into this hole Guericke introduced the nozzle of his empty globe, which
was stopped by means of a cock. On turning the cock, the air in the
cylinder rushed into the empty globe, or rather it was driven in by the
force of the outer air. So great was this force, that the twenty strong
men holding the ropes were pulled violently forward and were quite
unable to keep the piston up. A somewhat similar arrangement was used
for firing shots without powder.
It is remarkable that all this time Guericke had not heard of the
wonderful experiments made in Italy by Torricelli, the pupil of
Galileo. Had he done so, he might have been led to try to obtain a good
vacuum by means of mercury, as is done nowadays. But in some respects
his apparatus was greatly superior to that used by the Italians. In
order to find whether animals could breathe in rarefied air, some of
the Italians brought birds and other small animals into the vacuum at
the top of the column of mercury. It was found, of course, that the
animals could not live, partly because air was necessary for breathing,
and partly because their whole structure and constitution was
disturbed by the pressure of the air within them, which was no longer
counterbalanced by the pressure outside. Such experiments as these,
together with others on combustion, could be performed much better with
the air-pump as constructed by Guericke; although it must be borne in
mind that such experiments on animals are not now necessary, since we
know that animals cannot survive after they are deprived of air.
The effect of the invention of the air-pump was very great. Men
realised for the first time that they lived at the bottom of an ocean
of air; and that if all air were removed, the world would be quite
different. It would be a world in which neither animals nor human
beings could live. The air-pump in the course of the next century led
to two other great inventions, the steam-engine and the balloon.