Cystin has been used as an analytical reagent with satisfactory results for the estimation of copper. cadmium. cobalt, nickel and zinc, as well as for the separation of the latter three elements from calcium, barium and magnesium. Attempt to separate zinc from iron and aluminium in the presence of tartaric or sulphosalicylic acid in ammoniacal solution led, however, to no fruitful result;. The precipitation of zinc under these circumstances was considerably retarded, or even more or less completely inhibited, depending upon the amount of iron or of sulphosalicylic acid present, though tartaric acid or sulphosalicyclic acid alone has got no effect.