Abstract
The effects of various surface treatments on the plastic properties of zinc crystals are examined. Roscoe's observation of the hardening effect of a surface film of oxide is confirmed; electrolytically polished specimens have critical shear stresses of about 33 gm mm-2 whereas oxidized specimens have values up to 65 gm.mm-2. Similar effects are observed on the rate of creep under constant stress The Rehbinder effect - softening crystals by immersing them in paraffin and oleic acid - can be produced on oxidized specimens but not on others. Oxidized specimens do not respond instantly to immersion and the time of response is proportional to the viscosity of the liquid. It is suggested that the liquid penetrates the oxide film and reduces its ability to harden the metal; the action of the oxide film itself is not yet clear.