432 JOSEPH BARRELL 



and yet be brittle because of a low elastic limit under rapid tensile 

 stress, combined with lack of plasticity at ordinary temperatures. 

 At temperatures sufificiently high, the modulus is not greatly 

 different, but the elastic limit is still lower. The substance is now, 

 however, plastic, rather than brittle, since plasticity is greatly 

 increased; but a rapid strain, exceeding the rapidity with which 

 plastic deformation can take place, may still produce fracture. 

 Another substance, such as rubber, may have a low modulus of 

 elasticity and yet a relatively high elastic limit. 



Among similar substances under similar physical conditions there 

 is, however, a definite association of these properties which for the 

 metals is brought out well in a tabulation by Johnston and L. H. 

 Adams.' It is shown for a class of substances, such as the metals, 

 that the modulus of elasticity, the hardness, the tensile strength, and 

 the elastic limit all, so far as the data are given, occur in the same 

 order; so that of two metals that which has the higher elastic limit 

 is the higher also in the other quahties. From this association 

 there results a ready mental confusion between rigidity and 

 strength. The one, however, denotes the degree of resistance to 

 distortion from a unit-shearing stress and gives the modulus of 

 ridigity. The other is measured by the elastic limit. As an 

 example of the confusion between these two different properties, 

 it is known that the earth as a whole is more rigid than steel. This 

 to many would appear to mean that it was stronger than steel. 

 Earthquake waves show that the earth becomes progressively 

 more incompressible and more rigid with depth. This might be 

 held as evidence against the existence of a thick sphere of weakness, 

 the asthenosphere. High incompressibility and high rigidity are 

 not, however, direct testimony of strength, and it is the purpose 

 of the next topic to show under what conditions a solid may be very 

 rigid and yet very weak. 



CONDITIONS FAVORING ASSOCIATION OF HIGH RIGIDITY WITH LOW 



ELASTIC LIMIT 



Alpine glaciers as well as the Alpine-like margins of the Green- 

 land ice sheet move much more rapidly in the summer than in the 



I "On the Effect of High Pressures on the Physical and Chemical Behavior of 

 Solids," Amer. Jour. Sci., XXXV (1913), 220. 



