Specific examples of preferable water-soluble organic solvents usable in the present invention include alkyl alcohols having 1 to 4 carbon atoms, such as methyl alcohol, ethyl alcohol, isopropyl alcohol, n-propyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol and isobutyl alcohol; amides such as dimethylformamide and dimethylacetamide; ketones and keto-alcohols such as acetone and