10£ Dr. R. D. Kleeman on the Kinetic 



we may suppose (as is usually done) that the molecules 

 consist of three sets moving at right angles to one another. 

 Therefore, if n 2 denotes the number of molecules that 

 per second a plane 1 cm. 2 in area, which is at right ang] 

 the motion of the molecules in a set, we have 



N 



or 

 or a., = -f— 



since Nm =j5, where m a is the absolute mass of a molecule 

 and p the density of the gas. Now the equation of state of a 

 perfect gas is 



RTp 



P = 



in 



where /; is its pressure in dynes and T its temperature, m is 



the mass of a molecule relative to hydrogen, and K is a 

 constant whose value is S'2ij x 1G 7 . But we also have 



p = "2 A, 

 and hence 



gi M6X10W oi . A = 2 . 478xl0 , T ^. 



m a o III mv 



From the kinetic theorv of gases, we have 



and thus 



_?>j, _ 3RT 

 p in 



in v J in 



= 2-534X10- 30 s/Trn, 



on substituting l/ol x 10~' 11 Eor — , the absolute ma 



in 



the hydrogen atom determined by Rutherford From expe- 

 riments on the * particle. 



It follows From the above that it' n a molecules cross a plane 

 1 cm. 1 in area per second at right angles in one direction 

 in a substance of any density, the pressure these molecules 

 produce Is n 8 A. Bui since the molecules in a Bubstance cross 

 the plane in ;ill directions, it will be oecessarj to investigate 

 the pressure they produce in terms of the total numbei // 

 passing through the plane Erom one side to the other. Take 



