CHAPTER VII. The Air-Pump

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.

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.

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.

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.

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.