“…Fractional precipitation with ammonia has long been a favorite procedure (32). Thus, it has been employed for the purification of lanthanum (46,52,82,83,84,88,135,143,370); for the separation of the cerium earths in general (89,105,106,109,339,340,342,397,437); for the separation of the yttrium earths in general (19,95,96,104,107,118,170,261,266,308,309,332,335,336,337,397,405); and for such specific separations as gadolinium from samarium (16,24,109, 155, 261), holmium from erbium (25,174,183), didymium from cerium (49), didymium from lanthanum (83,86,129,143,176), samarium from didymium (90, 102), yttrium earths from yttrium (107, 108, 118, 182, 397), other earths from erbium (168), dysprosium from terbium (193), terbium from gadolinium (291, 402), and praseodymium from lanthanum…”