Abstract
Experiments in this laboratory have shown that light and other forms of radiant energy exert a marked influence on the rate of wandering in the electric current. Thus the rate of wandering of arsenic sulphide suspensions is about twice as great in the dark as under strong illumination in the sun, or in the carbon arc or in the Nernst lamp. On the other hand, the rate of wandering of mastic emulsions is increased under strong illumination, and that to the extent of about forty per cent. of the value in the dark. Ferric hydroxide is retarded in the light to the extent of about six per cent., while chlorophyll suspensions are accelerated in the light to the extent of about forty per cent. The effect of the radiant energy is not in general instantaneous, but requires a few minutes exposure to reach its greatest value. In general also the effect is reversible, that is colloids whose rates of wandering have been influenced by radiant energy, return to their original values if kept in the dark for some minutes.
These phenomena are very interesting in connection with photochemical reactions in general. Arsenic sulphide suspensions may be kept almost indefinitely in the dark. On exposure to light they oxidize rapidly. Knowing now that light also reduces the negative charge on the colloidal particles of arsenic sulphide, one can arrive at a comparatively simple electrical theory of photooxidation. If one assumes that oxidation takes place when two hydroxyl ions can give up their charges, forming water and nascent oxygen available for oxidation, and that subsequently the negative charges thus set free may react with free hydrogen ions and free oxygen to form new hydroxyl ions which may again discharge and oxidize, it will be readily seen that the rate of oxidation will primarily depend on the value of the negative charge which the oxidized particle itself carries.
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