(_c_) A solution of the salts is partially precipitated by ammonia; that is, the solution is mixed with a small quantity of ammonia insufficient for the precipitation of the entire quantity of the bases (fractional precipitation). Thus, the didymium hydroxide is first precipitated from a mixture of the salts of didymium and lanthanum. A partial separation may be effected by repeating the solution of the precipitate and fractional precipitation, but a perfectly pure product is scarcely attainable. (_d_) The formates having different degrees of solubility (lanthanum formate 420 parts of water per one of salt, didymium formate 221, cerium formate 360, yttrium and erbium formates easily soluble) give a possible means of separating certain of the gadolinite metals from each other by a method of fractional solution and precipitation, as Bunsen, Bahr, Clève, and others have pointed out. (_e_) Crookes (1893) took advantage of the fractional precipitation of alcoholic solutions of the chlorides by amylene, and by this means separated, for example, erbium, terbium, and others. (_f_) Lastly, oxide of thorium ThO_{2} (Chapter VIII., Note 59) is separated by means of its solubility in a solution of sodium carbonate. A good method of separating these metals is not known, for they are so like each other. There are also only a few _methods of distinguishing_ them from each other, and we can only add the following four to the above. ^a The faculty of oxidising into a higher oxide. This is very characteristic for cerium, which gives the oxides Ce_{2}O_{3} and CeO_{2} or Ce_{2}O_{4}. Didymium also gives one colourless oxide, Di_{2}O_{3}, which is capable of forming salts (of a lilac colour), and another, according to Brauner, Di_{2}O_{5} which is dark brown and does not form salts, so far as is known, and (like ceric oxide) acts as an oxidising agent, like the higher oxides of tellurium, manganese, lead, and others. Lanthanum, yttrium, and many others are not capable of such oxidation. The presence of the higher oxides may be recognised by ignition in a stream of hydrogen, by which means the higher oxides are reduced to the lower, which then remain unaltered.