COLLECTION OF GASES 



45 



3. If an elastic balloon holding 1 cu. ft. of gas at the earth's surface 

 were to ascend until the barometer height is 15 in., would the volume 

 of the gas in the balloon grow smaller or larger? Why? 



4. Pouring a little water upon the piston of a dry pump is called 

 "priming" the pump. Why does this help the action of the pump? 



5. Explain why you can "suck" lemonade through a straw or other 

 tube. If the lemonade had the density of water, and you had the 

 necessary strength, how high could you suck it? 



45. Collection of Gases. When we wish to collect a 

 gas for study, we must remember that so-called " empty " 

 vessels are filled 

 with air, and that 

 to fill a vessel with 

 another gas we 

 must have some 

 way of removing 

 the air. One way 

 is to "sweep out" 

 the air by passing 

 the other gas 

 through the vessel for some time. Fig. 37 shows how 

 we might fill a bottle with illuminating gas by sweeping 

 out the air. We must use a large excess of the gas to 

 make sure no air remains. 



FIG. 37. 



Collecting Gas by Displacement of Air and Air by Dis- 

 placement of Water. 



A better way to get the air out of the collecting vessel is to fill the 

 vessel with water. Then we close the mouth of the full bottle with the 

 hand, or with a piece of cardboard, and set it, upside down, in a pan of 

 water (Fig. 37). We must have the mouth of the bottle under water 

 before we uncover it, or, as we well know, the water will fallout, and 

 air will take its place. Now, if we wish to collect a bottle full of the 

 air from the lungs, we can "blow" through a tube the end of which is 



