HYDROGRAPHY 



165 



In falling a little over 20 centigrade degrees the viscosity is 

 nearly doubled. At, say, 500 fathoms (the lower limit of 

 the photic zone) in the tropics, where the temperature is, say, 

 40° F., the viscosity is twice as great as at the surface, where 

 the temperature is, say, 80° F. Therefore planktonic organ- 

 isms would sink twice as fast at the surface as at 500 fathoms, 

 and consequently, to meet this difficulty, some of them have 

 developed devices to increase their surface resistance, or 

 others to diminish their specific gravity, such as oil globules 

 and gas bubbles, and increase of branched or flattened 

 appendages, along with a general reduction in size and weight. 

 Polar animals obviously do not require these adaptations to 

 rapid variations in viscosity so much as those inhabiting the 

 warmer seas, and consequently " suspension organs " are 

 more characteristic of the latter. 



It wiU be noticed from the table above that the specific 

 gravity of the water also increases somewhat with the decrease 

 of temperature in deeper water. This, along with the 

 increase of viscosity may be a help to a slowly sinking 

 organism in delaying its progress downwards. 



Alkalinity. 



The general alkalinity of sea-water is due to the presence of 

 the hydroxides of magnesium and calcium, but the degree of 

 alkalinity varies greatly from time to time and from place 

 to place, and depends to some extent at least upon the 

 amount of free carbon dioxide present in the water. Our 



