62 THE ROYAL SOCIETY OF CANADA 



Zeleny and McKeehan^ have carefully verified the truth of this 

 expression for small spores, powders, and spheres of wax and mer- 

 cury. When we apply the expression to the movement of ice crystals 

 we can do so only approximately since these crystals are not spherical. 

 Nevertheless, it furnishes us with some idea of the size of a small ice 

 sphere which would rise slowly through water. 



For drops of water in air where ii= -00018 and r= -001 cm. 

 the velocity is only 1 -2 cms. per second. When the radius increases 

 to -01 cm. the velocity becomes 121 cms. per sec. which is considerably 

 larger. This explains why fog particles settle so slowly and rain 

 drops fall so rapidly. 



Ice being lighter than water the effect of gravity is of course to 

 lift it upwards. Taking [x for water at 0°C as -01793, the density 

 of ice as -9166 and that of water as 1 - then for an ice sphere of -01 cm. 

 radius, 



, 981 (.01)2 (.9166-1) 



V = - ^^ — - — ^ 



^ .01793 



which is very nearly 1 cm. per second. A sphere two-tenths of a 

 millimeter in diameter could probably be seen at close range in water, 

 and be practically suspended in the water. It is evident that very 

 small ice crystals can be regarded as having no buoyancy, although 

 their density is quite normal. Some actual experiments on spheres 

 and cylinders of ice would be of considerable interest. 



1 John Zeleny and L. W. McKeehan, Phys, Zeit. II, 78 (1910). 



