Page:Elementary Text-book of Physics (Anthony, 1897).djvu/122

108 the escape of bubbles from the end of the tube. If, now, the Jar be removed, diffusion outward occurs more rapidly than diffusion inward: the pressure within soon becomes less than the atmospheric pressure, as is shown by the rise of the water in the tube.

The laws of gaseous diffusion have been shown by Osborne Reynolds to be consistent with the kinetic theory of gases.

97. Strain and Stress.—In the discussion of the third law of motion (§ 26) stress was defined as the mutual action of two bodies. In the applications made of the third law up to this point the stress has been considered entirely with reference to the two bodies between which it acts; that is, it has been tacitly assumed that the action is immediate, or, as it is called, is an action at a distance. But in many cases the action between two bodies is manifestly not of this sort, but is due to the presence and action of intervening bodies. These intervening bodies, when looked at generally, are called the intervening medium. In these cases we may apply the third law of motion to the parts of the medium, and assert that there exists a stress between any two contiguous portions of the medium. This stress will vary from point to point and with the direction of the surface across which it acts, and also with the peculiarities of the medium. Experiment shows that the application of stress to a medium is always accompanied by a change of form or deformation of the medium. This deformation is called a strain.

In some bodies equal stresses applied in any direction produce equal and similar strains. Such bodies are isotropic. In others the strain alters with the direction of the stress. These bodies are eolotropic.

According to the molecular theory of matter, the form of a body is permanent so long as the resultant of the stresses acting on it from without, with the interior forces existing between the individual molecules of the body, reduces to zero. The molecular forces and motions are such that there is a certain form of the body