Examples of the organic solvent to be used in the dispersion bath include aromatic hydrocarbons and halides thereof such as toluene, benzene, chlorobenzene and dichlorobenzene; and aliphatic hydrocarbons, cycloaliphatic hydrocarbons and halides thereof such as n-heptane, n-hexane, liquid paraffin, cyclohexane, dichloromethane and dichloroethane.