chap. xxin. j COMPRESSIBILITY OF GASES. 271 



walls of the bladder by the stretching, comes to be 

 equal to the elastic force of the gas, when further 

 dilatation will cease. As soon as air is permitted to 

 enter the receiver the bladder becomes restored to its 

 former size. Owing to their constant tendency to 

 expand, gases have no definite volume. 



Compressibility of gas. A liquid has been 

 seen to be very little compressible. The slight com- 

 pression, however, to which liquid is subject develops 

 in it a very great force of reaction. Gas, on the other 

 hand, is readily compressible, and may be reduced to 

 one half its volume without developing a force greater 

 than that of the atmosphere. The compressibility of 

 gases is easily shown by means of a syringe closed at 

 one end and fitted at the other with an air-tight 

 piston. By pressing sufficiently on the piston, the 

 volume of air in the syringe may be reduced very con- 

 siderably. On removing the pressure, the reaction of 

 the gas will force out the piston. 



As gas is reduced in volume by pressure in this 

 way, it exerts pressure on the vessel or tube contain- 

 ing it, which increases as the volume diminishes. 



^7 7 



This is expressed by a law discovered independently 

 by Boyle and Marriotte, and called by their name. 



BoyJe's or Marriotte' s laiv. According to it, 

 the pressure of a given quantity of gas increases as its 

 volume is diminished, and vice, versa. Its pressure, 

 that is, is inversely proportional to its volume. Since 

 diminished volume means increased density, the law 

 may also be expressed by saying that the pressure of 

 a given quantity of gas is directly proportional to its 

 density. The experiments by means of which this 

 law was proved were made with an apparatus repre- 

 sented in Fig. 118. It consists of a bent tube, with a 

 short limb closed at its extremity A, and a long limb 

 open ate. Attached to both limbs is a scale, the divisions 

 of which mark equal capacities of the parts of the tube 



