156-158] The Size of Molecules 139 



The following table contains the values of the pressure coefficients of a 

 number of gases, as given in Travers' Study of Gases. 



Hydrogen* '0036624, 



Nitrogen* '00367445, 



Oxygen f "0036742, 



Air} -0036700. 



For an ideal gas the value would of course be ^ or '0036635. 



LJ i 3'U4 



Changes at Constant Pressure. 



157. We next imagine the pressure kept constant. Experiments con- 

 ducted under these conditions enable us to determine a "volume coefficient" 

 K V defined by 



so that the volume at temperature is given by 



v l = v Q (l + K V &). 

 By a method similar to that of the last section, we find 



.................. (327) . 



This is more complicated than the corresponding pressure-formula (325) 

 in that it depends on both the pressure and volume. 



Determination of Size of Molecules. 



158. From a series of experiments in which the values of K P and K V 

 are both determined, it will clearly be possible to calculate the values of both 

 a and 6. Calculations of this type will be found in Van der Waals' essay. 



The determination of b is of special importance, because from it we can 

 calculate the value of <r, the diameter of the sphere of molecular action. 



The values for b at which Van der Waals arrives from the discussion of a 

 great number of experiments by Regnault are as follows : 



Air -0026, 



Carbon dioxide || '0030, 

 Hydrogen IT '00069. 



The numbers apply to unit volume of gas at a pressure of one metre of 

 mercury. 



* Obtained by Chappuis, the pressure at C. being 1000 mm. 



t Von Jolly. 



J Mendeleef, from observations of Begnault, Magnus and Jolly. 



Continuity, p. 400. || ibid. p. 402. See also p. 413. H ibid. p. 401. 



