CHAPTER XIV 



ELASTICITY 



Protoplasm is elastic. This property in itself does not appear 

 to have so important a bearing on the physiological behavior of 

 protoplasm as does viscosity. It appears rather to be the out- 

 come of other more significant qualities. The chief interest in 

 the elastic property of protoplasm lies in its bearing on the 

 fundamental question of protoplasmic structure; elasticity is the 

 best indication that we have of the structure of living matter. 



Elasticity, in general, implies springiness and may be defined 

 as that property of substances which causes them to assume their 

 original shape after being deformed. 



If a steel anvil is struck with a hammer, the hammer rebounds, 

 and we say that the steel is elastic. An enclosed gas or liquid 

 when compressed assumes or tends to assume its original volume 

 when the pressure is released. The gas or liquid is elastic. 

 When rubber is stretched and of itself returns, it does so because 

 it is elastic. All three of these examples represent different 

 forms of elasticity in the technical or popular sense. Gases and 

 liquids possess an elasticity of volume; with this we are not 

 concerned here. If elasticity is defined and measured in terms 

 of the force necessary to produce maximum deformation from 

 which a body will recover when the force is removed, then the 

 greater the force and the more perfect the recovery the greater 

 is the elasticity; in this sense, the steel anvil is more elastic than 

 the rubber band. Rubber is of high extensibility; this is the 

 proper term when stretch is referred to. There is, however, a 

 type of stretch that does not involve recovery and, therefore, 

 elasticity; this is ductility. Lead is highly ductile and therefore 

 extensile, but it shows no (or very little) recovery and is conse- 

 quently not elastic. 



There are a number of other properties closely associated with 

 elasticity which are equally characteristic of protoplasm but are 



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