The flow of solids

The present widespread use of polymer plastics has made necessary the study of the mechanical properties of these substances and thus has focussed the interest of the engineer and the physicist once again upon the general problem of the flow properties of solid materials. This is a classical problem; over a period of nearly 100 years it has repeatedly made headlines in physics. The empirical foundations were laid by men whose names mark the development of classical physics—Maxwell, Hopkinson, Kohlrausch. Boltzmann and Wiechert developed a phenomenological, e.g. descriptive, theory which is still unsurpassed. The great Italian mathematician V. Volterra recognized the mathematical implications of this theory and therefrom established the theory of the integral equation that carries his name. The applied physicists and engineers joined forces, bringing with them the tools of modern technology and extending the field of experimental research to a point undreamed of some years ago. And now the theorists of modern physics begin to see a way in which the observed macroscopical behavior of matter may be explained in detail by a satisfactory molecular theory. But in spite of all this combined effort the problem is far from being completely solved.