ving from 1 to 6 carbon atoms, or wherein R and R; can combine to form a non-metal atom containing 5-, 6- or 7 -membered ring, and X is a hydrogen atom, a sodium atom, a potassium atom, a lithium atom, or an ammonium group, said acrylic acid-acrylamide copolymer being present in said sys-v tem at about 0.001 to about 0.1 parts by weight per part by weight of gelatin, and having a limiting viscosit