786 Prof. A. LI. Hughes on Dissociation of 



top of the McLeod gauge. It was decided, therefore, to 

 start at higher pressures in spite of the fact, that sometimes 

 neither set of gas laws would apply accurately : 



— = w — - =3*25 if " high pressure " gas laws apply. 

 iii 90 • . 



= 1'80 if " low pressure " gas laws apply 



\ = mean free path of an electron in the experimental tube 



at 90° K. This was taken as 4^/2 x m.f.p. of a 



molecule. There will be two values according as to 



whether the " high pressure " or " low pressure ; ' laws 



apply. Jeans gives 11'6 x 10" 6 cm. as the m.f.p. of 



the hydrogen molecule at 760 mm. pressure and 0°C 



The A-alues of "6" calculated for runs A, B, C, and D 



(Table I.) are given in Table III. For the initial pressures 



A and B ; only the " H.P."" values are admissible. 



Table III. 



j5 Number of molecules disappearing, 



a ues o ""Number of collisions between electrons and molecules. 



Accelerating; Potential = 71 yolts. 





A 



B 



C 



1 

 D 



Initial Pressure = 



5080 

 XlO-5 mm. 



2160 

 Xl0-p mm. 



942 

 X 10 -5 mm. 



200 

 X 10-5 mm. 



m.f.p. of H 2 f"H.P."law... 

 molecule \ " L.P." law.... 



•053 cm. 



•124 cm. 



. '286 cm. 

 •520 cm. 



1*30 cm. 

 2-45 cm. 



fl '„ /"H.P.'law 



, ° \ i! L.P."law 



•129 



•136 



•122 

 •168 



(•Ho) 

 •178 



The initial pressure in C is in the region where neither 

 law is strictly applicable. The real value of u b " will, there- 

 fore be between '122 and *168 for C. For the pressures used 

 in D, %( L.P." laws are evidently to be used and the "L.P. - " 

 value of " b " should be taken. 



It is seen that ' ; b" which measures the clean up per 

 collision, increases progressively as the initial pressure 

 decreases. At the lowest pressure, D, about 1 out of every 

 G collisions results in the disappearance of a molecule. Here 

 the conditions are the simplest, the electrons practically 

 never make two collisions, and the atoms resulting from a 

 collision with an electron have a good chance of reaching 

 the walls and condensing there. 



