BASIDIA AND THE DISCHARGE OF SPORES 21 



more delicate than my own, have shown that Stokes' Law for the 

 fall of small spheres of a certain range of size in air is approximately 

 true. 1 These authors measured the density, radius, and rate of 

 fall of minute spheres of wax, mercury, and paraffin, which were 

 obtained by spraying. The rate of fall, as actually observed for 

 spheres about the size of spores, was found to be in almost exact 

 accordance with the Law. 



MiUikan, 2 who has contributed much to the recent upbuilding 

 of the atomic theory of electricity, by employing a method even 

 more delicate than that of M'Keehan and Zeleny has shown that 

 Stokes' Law, while true within 2 per cent, for spheres with a 

 diameter of 0-001 cm. or 10 yn, i.e. for spheres about the size of 

 spores, becomes increasingly inexact for spheres of smaller and 

 smaUer diameter. Stokes' Law, in its original form, is represented 

 by the following equation : 



v- 2 e^,, a2 



O n 



LL 



The Law, as corrected by MiUikan, has the foUowing form : 



p 

 = 



a 



/. b \ 



1 - -) 

 \ a) 



- 

 9 fj, \ pa 



where V the terminal velocity, 



p the density of the falling sphere, 

 o- = the density of the medium, 

 g = the acceleration due to gravity, 

 a = the radius of the falling sphere, 

 yu, = the viscosity of the medium, 

 p = the pressure of the gaseous medium, 

 b = a constant (according to Millikan = 6-25 x 10~ 4 

 when p is measured in centimetres of mercury). 



Zeleny and M'Keehan measured the rate of fall of the dry spores 

 of Lycopodium, Lycoperdon, and Polytrichum, with the result 

 that the spores were found to fall much more slowly than 

 they should do according to Stokes' Law. Thus the spores of 



1 Zeleny u. M'Keehan, "Die Endgesckwindigkeit des Falles kleiner Kugeln 

 in Luft," Physikalische Zeitschrift, Ed. XI, 1910, pp. 78-93. 



2 R. A. MiUikan, The Electron, Chicago, 1917, pp. 88-122. 



