body. We shall find it impossible, so long as the air is prevented from es- 

 caping, to press the sides of the bladder together ; and if the bladder be sul/- 

 mitted to such severe pressure as may be produced by mechanical means, it 

 will burst before the air will allow it to collapse. 



That air will not allow the entrance of another body into the space where 

 it is present, may also be proved by the following experiment : 



Let A B, fig. 1, be a glass vessel open at the end A, and having a short tube 

 from the bottom, furnished with a stopcock C. Let D E, fig. 2, be another 

 glass vessel containing water. On the surface of this water let a small piece 

 of cork F float. Let the vessel A B, having the stopcock C closed, bo now 

 inverted ; let its mouth A be placed over the cork F, and let it thus be pressed 

 to any depth in the reservoir D E. If the air in A B were capable of permit- 

 ting the entrance of another body into the space in which it is present, the 

 water in the reservoir D E would now enter at the mouth of the vessel A, and 

 rising in it, would stand at the same level within the vessel A B as that which 

 it has without it. But this is not found to be the case. When the vessel A 

 B is pressed into the reservoir, the surface of the water within A B will be 

 observed still near the mouth A, as will be indicated by the position of the 

 cork which floats upon it, and as is represented in fig. 3. It appears, there- 



Fig. 



fore, manifestly, that whatever be the cause, the water is excluded from the 

 vessel A B. That this cause is the presence of the air included in the ves- 

 sel, is proved by opening the stopcock C, and allowing the air to escape. By 

 the established principles of hydrostatics, the surface of the water within the 

 vessel A B exerts an upward pressure proportionate to the depth of that sur- 

 face below the surface of the water exterior to the vessel A B. This pressure 

 acting upon the air enclosed in the vessel A B, forces it out the moment the 

 stopcock C is opened, and immediately the surface of the water within A B 

 rises to the level of the surface without it. 



We have stated that the surface of the water within A B remains nearly at 

 the mouth of that vessel when it is plunged in the reservoir. It would remain 

 exactly at the mouth if air were incompressible ; but, on the contrary, this fluid 

 is highly compressible, allowing itself to be forced into reduced dimensions by 

 the application of adequate mechanical force. It is necessary, however, not 

 to confound compressibility with penetrability. So far from these qualities 

 being identical, the one implies the absence of the other. A body is compres- 

 sible when the forcible intrusion of another body into the space within which 

 it is confined causes its particles to retreat and to accommodate their ar- 

 rangement to the more limited space within which they are compelled to 

 exist. , 



The very fact of their thus retreating before the intruding body is a distinct 

 manifestation of their impenetrability. If they were penetrable, the body 



