H. If. Deelep — Viscous Flow of Glacier-Ice. 409 



In treating; of viscosity we are at the outset met by a verbal 

 difficulty. Viscosity and plasticity have both been used to denote 

 the same physical property. Some writers speak of ice and pitch 

 as being plastic as well as viscous. It is better, however, to follow 

 Maxwell,^ who defines a viscous body as one which undergoes con- 

 tinuous change of form under stresses, however small. All viscous 

 substances are on this view true liquids. A plastic substance, on the 

 other hand, only undergoes continuous change of form when the 

 stress exceeds a certain value. When the stress required to cause 

 continuous distortion is small, the substance is called a soft solid. 

 Until this limiting stress is reached the substance is elastic, and the 

 stress at which what is known as perfect elasticity ceases to be 

 obtained, and the substance undergoes permanent deformation, is 

 called the limit of perfect elasticity. 



We must also recognize that there are hard solids. In them, when 

 the stress exceeds the limit of perfect elasticity, the substance begins 

 to yield more or less steadily as the stress increases. In some cases 

 after this limit is reached the solid suifers a sudden permanent 

 elongation. It afterwards regains its strength, and then only yields 

 further with increasing stress. 



The point at which the material gives way is called the yield 

 point, and as it coincides very nearly with the limit of perfect 

 elasticity, and is easily detected, it is now always noted by pi'actical 

 engineers. But the distortion of a har-d solid is not produced under 

 the conditions which give rise to viscous flow. Indeed, the action 

 of the stress often appears to produce an allotropic change in the 

 material. The whole subject is a most difficult one, and has by no 

 means been completely worked out. There is no real boundary 

 separating a hard from a soft solid. The soft solid under a suffi- 

 ciently great stress yields much as a viscous fluid does ; whilst the 

 hard solid often fractures, and definite shear planes or faults are 

 formed. 



The difference between plasticity and viscosity is one of 

 fundamental importance to the engineer, for as long as the 

 stress put upon a foundation does not exceed the yield point 

 the structure raised upon it is permanently stable. It is quite 

 otherwise with a viscous material. No possible foundation can 

 be stable upon such a substance unless it be made to float like 

 a ship upon water; for, however large the area upon which the 

 stress acts, the foundation will yield continuously. 



With increase of temperature the yield point and limit of perfect 

 elasticity grow smaller and smaller in many instances ; and a soft 

 solid or plastic material may, with rise of temperature, become 

 a viscous liquid. 



By many the difference between the plastic and viscous condition 

 is not properly appreciated. We have our softer rocks spoken of 

 as though they wei'e viscous, and various theories of soil-cap motion 

 propounded to explain phenomena resulting from other agencies. 



1 " Theory of Heat," 1894 edition, p. 303. 



