"I want you to think about our experiment with the tumbler inverted in the basin of water," said Mr. Wilson. "That will help us to learn something new this morning. But first of all, you must tell me what you saw while I held the tumbler in the water.""The water stood in the inverted tumbler," said Fred. "None of it ran out. It was kept there by the downward pressure of the air on the water in the basin.""Quite right," said Mr. Wilson. "See, I will repeat the experiment with this test-tube. The water stands in it as it did in the tumbler.
"Suppose we now try it with another liquid-mercury. I will fill the tube with mercury, close it with my finger, and invert it in this little bowl of the same liquid. The mercury, you see, stands in the tube just as the water did, and for the same reason. It is held up by thedownward pressure of the air.
"Now let us try it with this long glass tube. I will fill the tube with water and invert it in the bowl of water as before. None of the water flows out.
"Let us try the same thing with mercury. I fill the tube, close it with my finger, and invert it in the usual way in this little bowl of mercury. What has happened?""Some of the mercury has run out of the tube into the vessel. There is no mercury in the top of the tube.""What is there, then? Air? There can be no air in that upper space,because we filled the tube with mercury, and the mercury has simply dropped down. It is an absolutely empty space. We call it a vacuum.
"Let us see whether the mercury will fall any lower in the tube. No, we cannot make it fall, so long as we keep the lower end in the vessel of mercury. If we take the tube out, all the mercury at once runs down it, but if we refill it and invert it again, the mercury will stand at the same height as before.
"Come to the front, Fred, and hold the tube steadily in the bowl. I will measure with a tap the column of mercury, from the top to the level of the liquid in the bowl below. It measures you see, as nearly as possible thirty inches.
"We must find out what all this means. But let us first learn why the mercury stands at all in this tube. We have here a long thin column of mercury thirty inches high. The mercury is a very heavy liquid metal, and its weightpresses downwards. If it could it would run down the tube into the bowl below. As it does not run out, there must be something to prevent it.
"Try and picture a very long tall column of air, of exactly the same thickness as the mercury in the tube, but reaching many miles upwards to the very extreme limit of the atmosphere. Imagine, if you like, that this long thin column of air is contained in a tube, reaching all the way up from the surface of the mercury in the bowl.""This long column of air then presses downwards, by reason of its own weight, upon the surface of the mercury in the bowl.
"What else is pressing down upon it as well as this column of air? The mercury in the tube is also pressing down upon it; it is trying to get out of the tube and cannot. The pressure of the air prevents it. The air and the mercury in the tube press down with equal force, and balance each other like weights in a pair of scales."Lesson 37
