CHAPTER III. 



EQUILIBRIUM AND PRESSURE OF GASES. ELASTIC 



FLUIDS. 



40. Elasticity of Gases. The pressure of an elastic 

 fluid is measured exactly in the same way as the pressure of 

 a liquid (Art. 6), and the equality of pressure in every direc- 

 tion, and of transmission of pressure, are equally true of 

 liquids and gases (Arts. 7 and 8). There is, however, this 

 difference between a liquid and a gas : when a liquid is con- 

 fined in a vessel, no pressure is exerted against the sides 

 except that which is due to the weight of the liquid itself, 

 or that which is transmitted by the liquid from some point 

 on the surface at which, an external force is applied; 

 whereas, if a gas be contained in a closed vessel, there is, 

 although modified by the action of gravity, an outward 

 pressure exerted against the sides, which is due to the elas- 

 ticity of the gas, and which depends upon its volume and 

 temperature.* It is therefore evident that generally a gas 

 cannot have a free surface like a liquid (Art. 11), for such 

 a surface implies that at each point the pressure is nothing, 

 *. e., if it be covered by an envelope everywhere in close con- 

 tact with it, no pressure is exerted against the envelope. It 

 is also evident that, if a portion of the gas be withdrawn 

 from the vessel, that which remains will not fill the same 

 part of the vessel that it occupied before, as in the case of a 

 liquid, but will expand so as to fill the whole vessel, press- 

 ing, but with diminished force, against its sides at every 

 point (Art. 2). From this property of gases, they are called 

 elastic fluids ; the outward pressure which a gas exerts 



* If the gas is not confined within a limited space, the effect of its elasticity 

 might be the unlimited expansion and ultimate dispersion of the gas. 



