ATOMIC HYDROGEN 111 



sion in the liquid, upon a rotating table, the precipitate will collect in a 

 ring as far from the center as possible, although the relative angular motion 

 of the beaker and its contents is the same as before. A little study proves 

 that the phenomena are due to unbalanced centrifugal forces. For example, 

 when the liquid is stirred to set it in rotation, centrifugal force produces 

 a greater hydrostatic pressure near the walls of the beaker. But the liquid 

 very close to the bottom surface of the beaker, because of friction, cannot 

 rotate so fast, and therefore the centrifugal force is not so great and does 

 not counteract the radial hydrostatic pressure difference existing in the 

 upper layers. The liquid in contact with the glass bottom is thus forced 

 inwards and carries the precipitate with it. 



The phenomena connected with the formation and the disappearance 

 of ice in a large lake, such as Lake George, have interested me for years. 

 One clear night at the end of December, when the water of a large bay was 

 at a uniform temperature of not over 0.2° C. and the air temperature was 

 ^22° C, ice, which formed slowly at some places on the shore, melted in 

 a couple of minutes when pushed out a few meters from the shore. There 

 was no wind in the bay, but a slight breeze over the central part of the lake 

 caused a very slow circulation of water in the bay with a velocity of 

 perhaps i or 2 cm. per second. 



In contrast with this consider the phenomena observed one clear 

 afternoon of the following April. The body of the lake was still covered 

 with ice, which was about 20 cm. thick, but close to the shore there were 

 places where the ice had melted back for a distance of 5 meters or more. 

 Although the air temperature was +3° C. and the water 10 cm. below 

 the surface was at +2.5° C, ice crystals about 50 cm. long formed in 

 these pools in less than half an hour. After considerable analysis I believe 

 I can explain this apparent paradox by the stability in the stratification of 

 the water in April caused by the denser underlying warm water which had 

 been heated by the sun. With this stability which prevented vertical con- 

 vection the surface water could freeze because of the radiation into the 

 clear sky. But in December the water temperature was so uniform that 

 the differences of density were not sufficient to prevent verticle circulation, 

 and thus the surface could not cool to the freezing point. It appears, then, 

 that a pool of water at +1° C., exposed to cold air with a slight wind, can 

 be made to freeze more rapidly if the water is heated from the bottom. 

 Sometime I want to try this as an experiment. 



All hobbies, however, stimulate individual action, and many develop 

 wholesome curiosity. The child should acquire them early, and our 

 educational system should foster them. 



