Colloids in Additively Colored Sodium Chloride

A study has been made of the formation of colloids in additively colored sodium chloride. Both natural and synthetic single crystals were additively colored by heating in a vapor of sodium and by injecting electrons from a point cathode. Subsequent irradiation with ultraviolet light and heating of the synthetic crystals produced an absorbing type colloid throughout the crystal. The same treatment applied to the natural crystals produced $F$ centers throughout the bulk of the crystal and absorbing type colloid specks at localized points in the crystal. In the synthetic crystals the formation of these colloids is shown to be related to the presence of hydroxyl ions in the crystals prior to coloration. In the natural crystal the formation of such colloids is dependent upon the presence of small occlusions of water distributed randomly in the crystal prior to coloration.