GENERAL BIOLOGY 689 



various organisms, we must first of all become acquainted 

 with the external conditions governing floating and sinking ; 

 mainly owing to the investigations of Chun and Ostwald our 

 knowledge on this point has increased greatly in recent years/ 



First and foremost among these conditions is the specific Specific 

 gravity of ocean water. If an organism has the same specific fhg'^^^JJr^ 

 gravity as the sea-water it floats, because, according to the law 

 of Archimedes, it displaces a volume of water equal to its own 

 weight. When the specific gravity of the organism is greater 

 than that of the water it has a surplus gravity and may possibly 

 sink. If nothing counteracts its sinking, the velocity will be 

 proportionate to the value of the surplus gravity (equal to the 

 specific gravity of the organism minus the specific gravity of 

 the water). 



Experience shows, however, that all objects of the same viscosity of 

 specific gravity do not sink with equal velocity. Fine sand ^^^ '''^^"'• 

 particles float much longer in water than large pebbles, although 

 they have the same specific gravity. This is due to a 

 property more or less peculiar to all liquids, called the viscosity 

 or the internal friction of the liquid, but in a liquid with a 

 definite viscosity objects sink with varying velocity, which 

 depends on what has been termed the surface resistance of 

 bodies. 



An object has a great surface resistance, and sinks slowly. Surface 

 when its surface is largfe compared with its volume, and when "resistance 



r 1^ ^., 1 ,,., of bodies. 



Its surface presents a large area at right angles to the direction 

 of the sinking. 



Surplus gravity and surface resistance are the two properties 

 in sinking bodies which determine the velocity of their sinking. 

 The greater their surplus gravity and the smaller their surface 

 resistance the greater is the velocity of their sinking. High 

 specific gravity and great viscosity of the water counteract the 

 sinking and require lower specific gravity and less surface 

 resistance on the part of the organisms in order to keep them 

 floating. 



We will first consider the two " external conditions," the 

 specific gravity and the viscosity of the water, and then discuss 

 the faculty of regulating the surplus gravity and surface 

 resistance possessed by the organisms, enabling them to adapt 

 themselves to their surroundings. The importance of the two 

 elements, specific gravity and viscosity, anywhere in the ocean 



1 See, for instance, Chun's Reisebericht {loc. cit.); W. Ostwald, " Theoretische Plankton- 

 studien," Zoologische Jahrbiicher, Abtg. Systematik, etc., Bd. i8, Jena 1903 ; " Zur Lehre vom 

 Plankton," Naturwissenscliaftliche Wocheiischrift, N.F., Bd. 2, Jena, 1903. 



2 Y 



