Whilst the low atomic weights of scandium and yttrium place them, to some extent, apart from the other rare earth elements, the latter element at least is so closely allied in properties to the other members of the group that yttria is one of the typical oxides of the family. Scandium and its compounds, however, present many peculiarities of behaviour when compared with the typical members, on the grounds of which Urbain[426] has contended that scandia should not be classed among the rare earths at all. Whilst this contention is perhaps rather extreme, especially in view of the fact that in nature scandia always occurs with other yttria oxides, it must be admitted that in many respects the element is anomalous. The oxide is the weakest base of the whole group, yet the oxalate is comparatively readily soluble in mineral acids (compare p. 132), and the potassium double sulphate is almost insoluble in potassium sulphate. The sulphate is altogether exceptional in that it is very easily soluble in water, and crystallises out with 6 molecules of water of crystallisation. The fluoride and the carbonate both dissolve readily in excess of precipitant, whilst sodium thiosulphate precipitates a basic salt from neutral solutions. [426] _Chem. News_, 1905, ~90~, 319. Meyer has pointed out the close resemblance between beryllium and scandium. The oxide and salts are colourless; the latter have a peculiar sweet astringent taste, and readily yield basic salts. The _hydroxide_, Sc(OH)₃, is thrown down by alkalies as a bulky white gelatinous mass; the _oxide_ is a white powder, less readily soluble in dilute acids than most of the rare earths. The _fluoride_ is important on account of its insolubility in mineral acids, which exceeds that of all the other rare earth fluorides, and approaches that of thorium. It is thrown down from neutral or acid solutions by addition of hydrofluoric acid or a soluble fluoride; if the solution be boiled, a soluble silicofluoride will also precipitate scandium fluoride, though no precipitate is obtained in the cold. This behaviour is due to the ease with which the silicofluoride is hydrolysed at high temperatures, according to the equation: Sc₂(SiF₆)₃ + 6H₂O = 2ScF₃ + 3SiO₂ + 6H₂F₂ and is of great value in separating scandium from the other earths. The fluoride is extremely resistant to acids, being completely decomposed only by fused bisulphate. In the absence of acids, the freshly precipitated fluoride dissolves in excess of concentrated alkali fluoride, forming double salts; in this behaviour, scandium resembles zirconium, but differs from thorium and the cerium and yttrium elements.