Examples of suitable polyhydroxy compounds belonging to the third class include tris(hydroxyphenyl) methane compounds represented by the general formula ##STR2## in which R4 is a hydrogen atom or alkyl group having 1 to 4 carbon atoms, each R5 is, independently from the others, a hydrogen atom, halogen atom, alkyl group having 1 to 4 carbon atoms or alkoxy group having 1 to 4 carbon atoms, the sub