8o 



ELEMENTS OF SCIENCE 



the receiver and so the external pressure, antagonistic to 

 that of the air contained within the delicate vessel, 

 ceases, the vessel will be blown to pieces by the unop- 

 posed elastic force of -the air which it contained. 



Thus it is plain that any small portion of air so cut 

 off from communication with the atmosphere, still exer- 

 cises pressure (a pressure that decreases as the volume 

 becomes greater), which cannot be due to the weight of 



so small a portion of air, but 

 must arise from its expansive 

 force alone. 



As has been said, the laws of 

 pneumatics modify in various 

 degrees the actions of liquids. 

 Of two vessels, A and B(Fig.i5), 

 let A be filled with water up 

 to the level F, and B be empty. 

 Then let a bent tube CDE (the 

 limb DE being longer than the 

 limb DC) be filled with water 

 and temporarily closed (e.g., 

 with the finger) at either end. 



Next let the bent tube be so placed that the end C be 

 immersed in the liquid at A, while the other end, E, is 

 over the empty vessel B. Finally, let both ends of the 

 tube be simultaneously unclosed. Then as gravity brings 

 down the water in the limb DE it will be replaced by a 

 rise of water from A up the limb CD to replace it a 

 rise due to the pressure of the atmosphere on the surface 

 of the water in A. By the continuation of this process 

 the water in A will gradually become transferred into 

 the vessel B. Such an instrument is called a siphon. 

 The same action will result if the tube be placed in 

 a similar position empty of water, provided only it is 



