A model is proposed for a solid whose mathematical form is similar to the G
inzburg-Landau theory for superconductors. An effect involving loss of shea
r stability of a crystalline medium in a field of shear stresses is discuss
ed and the crystalline media are classified into those of the first and sec
ond kind depending on the nature of the defect penetration in the bulk of t
he material. (C) 1999 American Institute of Physics.