. . . . . . . . "oiling solvent is a compound represented by the following formula (III) or (IV): ##STR16## wherein R1 and R2 each represents an alkyl group having 1 to 4 carbon atoms, and X is a hydrogen atom, a halogen atom, a methyl group, an ethyl group, a methoxy group, an ethoxy group or an aryl group; ##STR17## wherein R3 and R4 each represents an alkyl group, with the proviso that the total number of carbo" .