The elements are divided into two chief families or groups, that of the cerium metals and that of the yttrium metals respectively. The cerium elements are separated by a process depending on the relative insolubility of their alkali double sulphates; in this group are included cerium, lanthanum, praseodymium, neodymium, and samarium. The yttrium family is further divided into four sub-groups: the first consists of scandium and yttrium; the second or terbium group of europium, gadolinium, and terbium; the third or erbium group of dysprosium, holmium, erbium, and thulium; and the fourth or ytterbium group of ytterbium and lutecium--the element celtium, recently discovered by Urbain, will also fall into this sub-group, but the discovery awaits confirmation. Whilst scandium and yttrium fall into somewhat abnormal positions, corresponding to their low atomic weights, the terbium elements occupy an intermediate position between the cerium elements and the remaining yttrium elements, or yttrium group proper, and so are frequently classified as a third or intermediate group. This list does not include all the names which have been put forward to designate what have been claimed from time to time as new elements; whilst the individuality of some of those included is not yet fully established, and the homogeneity of others has been called in question. The uncertainty is more pronounced among the yttrium elements than among the cerium elements; owing to the opportunities for investigation furnished by the commercial treatment of monazite, the chemistry of the cerium group may be regarded as complete. In the following table the elements are arranged in order of increasing atomic weight, and it can be seen at once how closely the division into groups follows this order: ELEMENT ATOMIC WT. COLOUR OF SALTS {Scandium, Sc 44·1 Colourless {Yttrium, Yt 89·0 Colourless {Lanthanum, La 139·0 Colourless {Cerium, Ce 140·25 Cerous, colourless; ceric, Cerium { orange to red Group. {Praseodymium, Pr 140·6 Green {Neodymium, Nd 144·3 Red to reddish-violet {Samarium, Sa 150·4 Topaz yellow Terbium {Europium, Eu 152·0 Faint rose Group. {Gadolinium, Gd 157·3 Colourless {Terbium, Tb 159·2 Colourless {Dysprosium, Dy 162·5 Bright green Erbium {Holmium, Ho 163·5 Yellow to orange Group. {Erbium, Er 167·7 Deep rose {Thulium, Tm 168·5 Bluish-green Ytterbium {Ytterbium, Yb 172·0 Colourless Group. {Lutecium, Lu 174·0 Colourless