DRIFTING LIFE 131 



oils formed in their bodies. These oils, being lighter than 

 water, tend to diminish the weight of the animals that 

 contain them. Those round globules of oil mentioned on 

 page 80 as distinguishing characters in the drifting eggs of 

 so many fishes probably tend to help in keeping the egg 

 suspended in the water (Plate 29). 



But of all the modes of obtaining buoyancy the indirect 

 methods of producing the same effect as a reduction in 

 specific gravity are the most wonderful. If we drop a stone 

 into water, and watch it sinking, we shall notice that its 

 sinking speed is considerably less than if it were falling 

 through air only. The speed is reduced by the frictional 

 resistance set up by the stone as it moves through the 

 water. This property of resistance to the movement of a 

 body is known as " viscosity." Some liquids are naturally 

 more " viscous " than others ; they are said to have a high 

 viscosity. Treacle is a very viscous fluid ; many oils also 

 are highly viscous. Thus the frictional resistance to a 

 falling body would be greater in treacle than in sea water. 

 Now, of course, the total frictional resistance experienced 

 by a falling body depends on the amount of area exposed 

 against the fluid through which it is moving. We know 

 that a slate takes longer to sink flatways than if it is on 

 edge. Clearly then, if the frictional resistance can be made 

 infinitely great compared with the actual weight of the body 

 itself, a stage will be reached at which it would counteract 

 the force due to gravity and the body would no longer 

 sink, but become suspended in the water. 



The structure of many plankton organisms shows an 

 attempt to increase the frictional resistance, which evidently 

 succeeds in keeping the organism almost suspended in the 

 water. Although they cannot completely counteract the 

 effect of gravity they can, nevertheless, come very near it 

 so that sinking is extremely slow. 



K 



