6. The permanence of the inequalities.
In spaces in which there are inequalities, owing to the presence or absence of a number of grains, in deficiency or excess of the number which would render the piling normal about local centres in the medium, these are permanent, as is now shown. And these are attended by inward or outward displacements and strains in the normal piling, as the case may be, extending indefinitely through the medium, causing everywhere dilatation equal to the strain but of opposite sign.
And, according to the evidence, the diameters of such inequalities are about 100-1000th of the wave-length.
When the arrangement of the grains about the centres is that of a nucleus of grains in normal piling on which the grains in strained normal piling rest, the nucleus, in normal piling, cannot gear with the grains in strained normal piling; so that there is a singular surface of misfit between the nucleus and the grains in strained normal piling.
Such singular surfaces are surfaces of weakness, and may be surfaces of freedom or surfaces of limited stability with the neighbouring grains.
