CHAPTER XV 



THE RATE OF FALL OF SPORES AND STOKES' LAW— APPENDIX 



So long ago as 1851 Stokes 1 published a paper called "On the 

 Effect of Internal Friction of Fluids on the Motion of Pendulums." 

 In the course of a mathematical treatment of his data, he deduced 

 an equation expressing the relations between the density of a 

 falling microscopic sphere, the size of the sphere, the velocity 

 of its fall, the density of the fluid through which it may fall, and 

 the viscosity of the fluid. The equation represents what is known 

 as Stokes' Law : 2 



9 M y ' 



where V = the terminaljvelocity, 



p = the density of the falling sphere, 

 o- = the density of the medium, 

 j/ = the acceleration due to gravity, 

 o = the radius of the falling sphere, 

 H = the viscosity of the medium. 



For more than forty years this equation remained untested 

 for the fall of small particles in air and other gases. This, no 

 doubt, was due to the technical difficulties of procuring microscopic 

 spheres of known density and size, and of dropping them through 

 gaseous media in such a manner that their rate of fall could be 

 measured. The verification of Stokes' Law by means of such 

 experiments has recently become of some importance owing to 

 the necessity of assuming it in investigations upon the electronic 

 charge as made by J. J. Thomson 3 with the cloud method. 



The only evidence hitherto 4 adduced to show that Stokes' Law 



1 G. Stokes, Camh. Phil. Trans., vol. ix., Part IT., p. 8. 



2 Cf. the Appendix to Chap. XVII. 



3 J. J. Thomson, Phil. Mag., December 1898; December 1899. 



4 This was written in 1907. Since then Zeleny and M'Keehan have recorded 

 experiments with lycopodium powder. Vide the Appendix. 



i6 4 



