PRESSURE OF A GAS 39 



of any gas varies directly as the absolute temperature if the 

 pressure remains constant. 



V T =F (l+aT). 



(3) The pressure of a given mass of any gas varies directly 

 as the absolute temperature, provided the volume of the gas 

 remains constant. 



P T =P Q (l+aT). 



Any one of these laws may be deduced from the other two. 

 The whole may be summed up in the formula 



PV=RT, 



where R is a constant varying only with the unit of energy used. 



TABLE V. 



UNIT OF ENERGY. VALUE OF R. 



Gram Centimetre - - 84,760. 



Joule (Volt Coulomb) - - 8-315. 



Volt Faraday - 0-8613 xlO*. 



Litre atmosphere - - 0-08207. 



Gram calorie - 1-985. 



The thermal equation with the gram calorie as unit is the one 

 most often employed in Biophysics, and is generally taken as 2. 

 That is, the gas equation assumes the approximate form 



PF=2T. 



What is the pressure of a gas ? The gas only manifests its 

 kinetic energy by alteration in its pressure or its volume. As 

 has already been stated, the molecules of a gas are free to move 

 in a straight line till they collide with another molecule or with 

 the walls of the containing vessel. The particle will then rebound 

 in its line of approach with a velocity equal to its orginal velocity, 

 but, of course, with the opposite sign. 



The pressure of a gas is due to the bombardment of the walls of 

 the containing vessels by the molecules. 



For unit volume, f of the total kinetic energy of the gas is 

 responsible for the pressure of that gas. Another conclusion 

 that may be drawn from the kinetic theory of gases is that the 

 velocity of a gaseous particle is inversely proportional to its mass. 



