164 



TENSION OF GASES. 



of the air within or without. TJie tension of aeriform 

 bodies supports the pressure exerted upon them, 

 and is equal to it. 



283. Experimental Illustrations of Tension. (1.) The 

 tension of confined air is well illustrated by the common pop-gun 

 It is also well illustrated by the common experiment 

 with bursting squares. These "squares" are made 

 of thin glass, are about two or three inches on each 

 edge, and are hermetically sealed under the ordinary 

 atmospheric pressure. The tension of the air within, 

 acting with equal intensity against the atmospheric 

 pressure from without, the frail walls remain unin- 

 jured. When, however, the "square" is placed 

 under the receiver of an air-pump and the external 

 pressure removed, the tension of 15 pounds to the 

 square inch is sufficient to burst the walls outward. 



(2.) Half fill a small bottle with water, close the neck with a cork 

 through which a small tube passes. The lower end 

 of this tube should dip into the liquid ; the upper 

 end should be drawn out to a smaller size. Apply 

 the lips to the upper end of the tube, and force air 

 into the bottle. Notice, describe, and explain what 

 takes place. 



(3.) Place the bottle, arranged as above described, 

 under the receiver of an air-pump, and exhaust the 

 air from the receiver. Water will be driven in a jet 

 from the tube. Explain. 



FIG. 98. 



FIG. 99. 



284. Mariotte's Law. TJie tempera- 

 ture remaining the same, the volume of 

 a given quantity of gas is inversely as the pres- 

 sure it supports. 



285, Experimental Verification of Mari- 

 otte's Law. This law may be experimentally verified 

 with Mariotte's tube. It consists of a long glass tube bent 

 as shown in Fig. 100, the long arm being open and the 

 short arm closed. A small quantity of mercury is poured 

 into the tube, so that the two mercurial surfaces are in the 



