PROPERTIES OF GASES 



61 



by that time the steam will have expelled all the air, then remove the flame 

 and close the clip immediately after. Holding the flask firmly, place it 

 under running water (note that the water in the flask boils). Wipe the 

 outside of the flask thoroughly and weigh. Open the clip and note the 

 inrush of the air. Weigh again. 



Find the volume of air in the flask by taking the volume of water remaining 

 in the flask from the volume of water required to fill the flask. 



Calculate the weight of 1000 c.c. Note the readings of the barometer 

 and thermometer. The weight of air under "standard" conditions, will 

 be found later. 



Air is a kind of matter. There is abundant evidence that air is 

 matter. It can be felt by the body as wind, and the results of its motion 

 are seen when it moves trees or loose objects on the ground. Even on 

 the calmest day the air can be felt by swinging the open hand to and fro. 



Air has weight. As air is a kind of matter it must have weight. 

 By weighing a flask from which air has been removed, and then again 

 after the admission of air, it is found that air has undoubtedly got 

 weight. Careful experiments show that under " standard " conditions 

 a litre of air weighs 1*293 gm. 



26. PEESSUEE EXEETED BY THE ATMOSPHEEE. 



i. Take a U-tube such as shown in 

 Fig. 48, and half fill it with water. 

 Attach a piece of rubber tubing to one 

 limb, and suck air out. Notice that air 

 rises in that limb. 



If a good air pump is available, repeat 

 this experiment, using a large U-tube 

 made by connecting two three-foot 

 lengths of glass tubing by means of 

 pressure tubing. Half fill the tube with 

 mercury and connect one nmb to the air 

 pump. Remove as much air as possible. 

 Note that it is impossible to get the 

 mercury to rise above a certain point. 



ii. Place in water one tube of the Hare's 

 apparatus for determining density (Fig. 

 49), and the other tube in mercury. 

 Suck out the air. The liquids rise. 

 Why ? Notice the difference in the 

 level of the mercury and the water. 

 Explain the cause of this difference. Let 

 one of the tubes of the Hare's apparatus 

 be much wider than the other. Place 

 the ends of the tubes in mercury, and 

 suck out the air. Is there any difference 

 in the height of the mercury in the large 

 and small tubes ? FIG, 48. 



