[105] _Phil Mag._ 1831, [ii.], ~10~, 139. [106] _Amer. J. Sci._ 1837, ~32~, 162. [107] _Pogg. Ann._ 1840, ~49~, 223. The question of the manner in which the thorium is combined in monazite is of considerable importance, in view of the fact that it is to this element that the mineral owes its commercial value. The amount present varies from traces up to over 20 per cent., but the usual value is between 5 and 7 per cent. The first explanation of its presence was advanced by Dunnington[108] who suggested, on the result of only one analysis, that orangite (ThSiO₄) was present mechanically mixed with the monazite. Penfield[109] supported this suggestion, and stated that in three analyses of pure material he found the ratio of rare earths to phosphorus pentoxide and that of thoria to silica exactly equal to unity, though the actual amounts of thoria varied considerably. He also quotes an analysis made by Rammelsberg in 1877, in which no thoria was found, to show that it is not an essential constituent. In a microscopic examination he found dark resinous particles scattered throughout the section; after moistening with hydrochloric acid, warming, and washing, these dark spots became white, and could be stained with fuchsine, the monazite remaining unaffected throughout. He concluded that these particles were thorite or orangite. [108] _Amer. Chem. J._ 1882, ~4~, 138. [109] _Amer. J. Sci._ 1882, [iii.], ~24~, 250; 1888, ~36~, 322. Blomstrand[110] disputed Penfield’s conclusions. In twelve analyses of monazite from various parts of Scandinavia he never once found either thoria or silica absent. Of these twelve analyses, two give the ratio of thoria to silica, ThO₂ : SiO₂, exactly unity, in seven cases the ratio is not greater than 1·25, in five cases it varies considerably. He summed up his results in three statements: (_a_) Silica is never absent; its amount depends not on the amount of thoria, but on the amount of phosphorus pentoxide present. (_b_) The thoria which is always present is combined partly with silica, partly with phosphorus pentoxide. (_c_) In most cases, the rare earths alone are insufficient to satisfy the ratio R₂O₃ : P₂O₅ = 1. [110] _J. pr. Chem._ 1890, ~41~, 266. An exhaustive examination of the question has been made more recently by Kress and Metzger.[111] They made in all over fifty analyses, using thirty different specimens of monazite; they estimated silica both as quartz and as silicate silica, and determined thorium by the fumarate method--the other investigators had used the thiosulphate method of Hermann (_vide_ p. 286). Their results may be summarised as follows: (i.) Silica is always present. (ii.) The amount of silica usually increases with the thoria, but not regularly. (iii.) By far the majority of cases showed insufficient total silica to combine with the thoria present.