英文科学读本 第四册·Lesson 48 The Syringe

Lesson 48 The Syringe

“We have learned a great many things in connection with the pressure of the atmosphere,” said Mr. Wilson. “Suppose we try to go a little further this morning. I have here a piece of glass tube open at both ends. If I close one end firmly with my finger, fill the tube under water, and invert it, you know the water will stand in the tube. Tell me why.”

“The pressure of the air on the surface of the water in the basin forces back the water into the tube, so that it cannot run out,” said Fred.

“Right. Now I remove my finger, and the water runs out at once. Why does it run out? There is air now pressing on the water in the tube with the same force as that which presses on the water in the basin. One pressure balances the other and the water runs out of the tube, and rests level with that in the basin.

“Come to the front, Fred, and suck the upper end of the tube. What do you find? Oh, you have a mouthful of water. How did that come about? Let us see. I have here a bottle filled with water. The cork fits very close, and I have passed a glass tube through a hole in the cork. Now I will press the cork tightly into the neck of the bottle, and you shall try again to suck up the water as before. What, can’t you do it? Try again. No, you can’t get any water this time, try as you will. The bottle is quite full of water, and the cork touches the top of it. There is no air in the bottle to press down on the water, as there was just now in the basin.

“This is enough to prove that you do not suck up the water. Now try again with the open tube, and you get a mouthful of water at once.

“The truth is that by sucking you merely remove the air from the tube. In the one case the pressure of the air on the water in the basin forced some of it up the tube when you removed the air. But in the other, there was no air in the bottle outside the tube, and, although you again drew away the air from the tube itself, the water did not rise. In both experiments, then, the water is not sucked up; it rises in the tube by the pressure of the air all round.

“Here is a glass squirt or syringe. I am going to show you how we can raise water with this instrument, and to make it clear I will use colored water for the experiment.

“But first of all let us take the instrument to pieces and examine it. It is simply a glass tube, with a rod, which we call the piston, fitting it closely all round, so that no air can pass between them. We say it fits air-tight.

“Now I will dip the end of the tube into the water, and raise the piston rod. As I draw up the piston the water rises in the tube. Why does it rise? That air-tight piston sucks up, and so removes the air from the tube, and an empty space or vacuum is formed above the water. The pressure of the air outside forces the water into the tube to fill up this vacuum.

“The water rises, then, in the piece of glass tube, and in the syringe, from the same cause. The air is first removed, and then the air-pressure all round forces the water up the tube.

“If we press the piston down again, the water is driven out in a stream.”