CONSTITUTION OP BODIES. G19 



that we can hardly see it, we are often inclined to call the substance a solid, 

 and even a hard solid. Thus the viscosity of cold pitch or of asphalt is so 

 great that the substance will break rather than yield to any sudden blow, and 

 yet if it is left for a sufficient time it will be found unable to remain in 

 equilibrium under the slight inequality of stress produced by its own weight, 

 but will flow like a fluid till its surface becomes level. 



If, therefore, we define a fluid as a substance which cannot remain in 

 permanent equilibrium under a stress not equal in all directions, we must call 

 these substances fluids, though they are so viscous that we can walk on them 

 without leaving any footprints. 



If a body, after having its form altered by the application of stress, tends 

 to recover its original form when the stress is removed, the body is said to 

 be elastic. 



The ratio of the numerical value of the stress to the numerical value of 

 the strain produced by it is called the coefficient of elasticity, and the ratio of 

 the strain to the stress is called the coefficient of pliability. 



There are as many kinds -of these coefficients as there are kinds of stress 

 and of strains or components of strains produced by them. 



If, then, the values of the coefficients of elasticity were to increase without 

 limit, the body would approximate to the condition of a rigid body. 



We may form an elastic body of great pliability by dissolving gelatine or 

 isinglass in hot water and allowing the solution to cool into a jelly. By 

 diminishing the proportion of gelatine the coefficient of elasticity of the jelly 

 may be diminished, so that a very small force is required to produce a large 

 change of form in the substance. 



When the deformation of an elastic body is pushed beyond certain limits 

 depending on the nature of the substance, it is found that when the stress 

 is removed it does not return exactly to its original shape, but remains per- 

 manently deformed. These limits of the different kinds of strain are called the 

 limits of perfect elasticity. 



There are other limits which may be called the limits of cohesion or of 

 tenacity, such that when the deformation of the body reaches these limits the 

 body breaks, tears asunder, or otherwise gives way, and the continuity of its 

 substance is destroyed. 



A body which can have its form permanently changed without any flaw 

 or break taking place is called mild. When the force required is small the 



782 



