CHARACTERISTIC EQUATION 15 



A. Colligative Relations, 

 i. Characteristic Equation. 



General. The preliminary problem as to the connexion 

 between properties and the circumstances of the substance 

 has first been followed out with success as regards volume, 

 and as is well known has yielded a satisfactory result for 

 dilute gases. 



The influences which change the volume (V), namely the 

 pressure (P) and temperature (7 in absolute centigrade 

 measure), are determined by the laws of Boyle and Gay- 

 Lussac, which are completely expressed by the equation 



PV= RT] 



this equation has moreover a theoretical basis in the kinetic 

 theory of gases, according to which the pressure, being due 

 to molecular collisions, is directly proportional to the 

 number of collisions, and therefore inversely to the volume, 

 whilst to explain the increase of pressure due to tempera- 

 ture, it is assumed that the kinetic energy of molecular 

 movement is proportional to the absolute temperature. 



The relation between volume and composition takes, in 

 accordance with this result, a very simple form, being con- 

 tained in Avogadro's law, according to which molecular 

 quantities under the same pressure and temperature occupy 

 the same volume, thus R is the same for all gases, provided 

 the molecular quantity be considered, as is shown by the 

 equation already given (Part II, p. 19) 



APV=*T, 



where A = , P is the pressure in kilograms per square 



metre, V the volume of a kilogram molecule in cubic 

 metres. 



This relation, which may also be based on the kinetic 

 theory, is, it is well known, so trustworthy that it serves 

 as a safe means of determining the molecular magnitude, 

 and was treated as such in Part II. 



