KINETIC THEORY 37 



pressure is the Vapour Pressure of the substance at that tempera- 

 ture. (Cf. tension of i^as in sohition.) 



In addition to the Kinetic Theory of gases, one must assume 

 the statement generally known as the Hypothesis of Avogadro : 

 " Equal volumes of different gases, at the same temperature and 

 pressure, contain the same number of molecules.''^ This proposition 

 has been adopted as a working hypothesis, and as such has stood 

 tlie test of time. It is, in fact, a necessary supplement to the 

 Atomic Theory. 



The laws governing the physical behaviour of gases are simple 

 statements correlating pressure, volume and temperature. 



(1) BoyWs Lazv. The volume of a given mass of gas varies 

 inversely as the pressure on it, if the temperature of the gas 

 remains constant. 



P 



(2) Charles'' or Gay Lussac's Law. The volume of a given mass 

 of any gas varies directly as the absolute temperature if the 

 pressure remains constant. 



Vr, = Fo (1 + O.T). 



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

 as the absolute temperature, provided the volume of the gas 

 remains constant. 



Vr, = Po (1 + O.T). 



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 



8-315 



0-8613 X 10* 

 008207 

 1-985 



Joule (Volt — Coulomb) 

 Volt — Faraday 

 Litre — atmosphere . 

 Gram — calorie 



The thermal constant w^ith the gratn caloric 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 



PV= 2T. 



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



