Soc. (C), 2424 (1969)) is dissolved in a solvent such as ethers including ethyl ether, isopropyl ether, tetra hydrofuran and 1,2-dimethoxyethane; hydrocarbons including hexane and heptane; and aromatic hydrocarbons including benzene and nitrobenzene, or a mixed solvent consisting of the above solvent and halogenated hydrocarbons such as methylene chloride, methylene bromide, 1,2-dichloroethane and