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A method has been devised by which the volume changes occurring during plastic flow in simple compression are directly measured in a dilatometer during the flow process. The great superiority of such a direct determination over an indirect determination from the alteration of dimensions is pointed out. Results are obtained for three rocks, quartz crystal, and a number of metals, including several grades of steel and iron. The volume change during plastic flow is not equal to the product of mean hydrostatic stress into the ordinary compressibility in the elastic range, but may vary in a much more complicated way. There are in the first place permanent changes of volume retained after release of stress; these have long been known and may be of either sign. In addition to the permanently retained changes there may be changes of volume during the action of stress which may be notably larger than the permanently retained part and which also may be of either sign. The most striking of these is a component of volume increase under increasing compressive stress which in the upper range of stress near the fracture point may be larger than the normal component of volume decrease, so that the total volume change is retrograde. This component is recoverable and reversible on release of stress. It would appear, therefore, that in at least some cases the irreversible phenomenon of fracture is prepared for in the region immediately below the stress of fracture by the appearance of a reversible change of volume of sign the opposite of that due to the elastic action of the stresses.