Gases, though generally lighter than air, all have a definite weight. This weight depends upon the volume of the gas and the pressure exerted, as may be proved by means of an instrument called a barometer (Fig. 53). The principle on which the barometer is based may be explained in the following manner.

If you put one end of a tube into a bowl of water and the other end into your mouth, you can draw the water up through the tube into your mouth by sucking. You may think that you suck the water up, but you do not; you merely suck the air out of the tube by means of the muscles of your mouth. The weight of the outer air then presses down on the water in the bowl and forces it up into the tube. As soon as you let the air into the tube again the water runs back into the bowl. If you had a tube 40 ft. in length and could suck all the air out of it, the water would rise up in the tube nearly 34 ft. It would stop at that height, because the weight of the column would just balance the weight of the air which presses down on the surface of the bowl. As the tube is more than 34 ft. long, in the space above the water, there would be nothing, not even air. Such a space is called a vacuum, from the Latin word meaning space without air. If you put the tube into a fluid lighter than water, such as alcohol, the alcohol will rise higher in the tube than 34 ft., because it will take more fluid to balance the weight of the air. If the fluid were heavier than water, as is quicksilver or mercury, it would not rise so high, because it would require less of it to equal and balance the weight of air.

Fig. 53. Simple Barometer.