Plasticity is a property possessed by solids which are capable of being moulded. Thus lead can be moulded into bullets without being first melted, and it is to the plasticity of clay that the potter's art owes its origin and developments. To explain the motion and behaviour of glaciers, ice was assumed to have the property of plasticity. This view was first put forth by Bordier of Geneva in 1773, and in 1841 Render founded a theory of glaciers on the same basis. He points out the fact that many observations seem to show that "glacier ice enjoys a kind of ductility which enables it to mould itself to its locality, to thin out, to swell, and to contract as if it were a soft paste." This theory of plasticity was later termed by Forbes the "viscous theory" of glacier motion. This theory has found many supporters, for ice has been shown in many cases to exhibit the property of plasticity in a striking manner. On the other hand, our ordinary experience of the extreme brittleness of ice has made it difficult for many to accept the idea.