398 Mr.. S. W. J. Smith on a 



If it be supposed that a collision occurs between a negative 

 ion and a molecule, when the ion approaches within a distance 

 R of the centre of the molecule, then R is determined by the 

 equation 7rR 2 n = N, where n is the number of molecules 

 per c.c. of a gas at a millimetre pressure. The following- 

 values of 10 8 x R in centimetres were found, taking n equal 

 to 2xl0 19 -f-760. 



The radius S of the sphere of action deduced from- ex- 

 periments on viscosity is given in the third line of the table. 

 The A*alues were obtained from the numbers for 7rS 2 N given 

 by Meyer (Meyer, < Kinetic Theory of Gases/ p. 300), N the 

 number of molecules in a c.c. of gas at 760 mm. pressure 

 being taken equal to 2 x 10 19 as in the above calculation 

 for R. 



Mean free path 

 N 



10 8 x R 



lirxS 



1 



A.ir. 



k. 



C0 2 . 



HOI. 



H 2 0. i 



079 



i 



•066 



•182 



•053 



•045 



1-36 



•82 



1 1-52 



1-64 



124 



1-72 



1-26 



207 



2-00 



1-99 



The quantity 2S is the smallest distance between the centres 

 of two molecules during a collision, and is not the same as 

 2R. The latter quantity is deduced from the maximum value 

 of a, so that in this case we are considering collisions which 

 would give rise to new ions if the negative ion were travelling 

 with a certain velocity. It appears from the above numbers 

 that a negative ion may pass inside the sphere of action of a 

 molecule with a high velocity without producing new ions. 



XXXIX. A Portable Capillary Electrometer. By 8. VV . J. 

 Smith, 21. A., Demonstrator m Physics, Royal College of 

 Science, London*. 



[Plate X.] 



THIS instrument is a modification of the form of capillary 

 electrometer represented in the first figure, and con- 

 sisting of two wide tubes joined across by a capillary tube 

 which is cylindrical, and may be horizontal or may slope up- 

 wards at any angle from b towards a. The apparatus contains 

 mercury and sulphuric acid of about maximum conductivity 

 distributed roughly as shown. A spring-key, like that repre- 



* Communicated by the Physical Society : read December, 12, 1902. 



