~Impregnation.~--The composition of the ‘Lighting Fluid,’ as the solution of salts used for impregnation is technically termed, varies slightly according to the nature of the mantle required, and the conditions of washing. It is of the greatest importance that the ratio of thoria to ceria should be constant and definite; the usual proportions are chosen so that the ratio of the oxides is 99 : 1. Fig. 11 shows at a glance to what a remarkable extent small variations in the percentage of ceria affect the luminosity of the finished product.[518] The thorium nitrate is made up with distilled water to a solution of 25-35 per cent. strength, and the calculated quantity of a standard solution of cerium nitrate is added. It is usual to add to the mixture a small quantity of another nitrate, which on ignition will leave an oxide of which the function is to strengthen the skeleton of ash. Beryllium, zirconium, magnesium, or aluminium nitrate is usually employed, in quantity calculated to leave an amount of oxide constituting about 0·5 per cent. of the total oxides; for ramie fabrics, beryllium nitrate is generally chosen. [518] Numberless theories have been advanced to account for the extraordinarily high light-emitting power of this particular mixture of thoria and ceria. An account of these would be beyond the province of the present work; the reader who desires to pursue the subject should consult the interesting work of Dr. H. W. Fischer, _Der Auerstrumpf_, Ahren’s _Sammlung_, 1906, vol. xi. _Vide_ also Lévy, _L’Éclairage à l’incandescence par le gaz_, Paris, 1910, Ch. II; and Foix, _Thèse présentée à la Faculté des Sciences de Paris_, Paris, Gauthier-Villars, 1910. The diagram is after Drossbach, _J. Gasbel_. 1898, 352. After having been immersed for two to five minutes in the solution, the separate lengths are freed from excess of the lighting fluid by means of a small wringing machine. The pressure between the rollers must be regulated very exactly, since on the amount of solution taken up by the fabric will depend the mass of the oxide skeleton. The weight of oxides left after ignition should be 0·5-0·6 gm. for a ‘normal’ upright mantle of 9·5 cm. length, corresponding to 1·0-1·2 gms. of the nitrates, or, for a 30 per cent. solution, to 3·3-4·0 gms. of solution. The weight of the fabric before impregnation is approximately 5 gms. for cotton, 3 gms. for ramie, and 1·5 gms. for artificial silk. A cotton mantle-fabric, therefore, must be allowed to retain rather less, a ramie fabric rather more, than its own weight of solution, whilst an artificial silk fabric must take up 2-2¹⁄₂ times its own weight of the fluid. The weight of the oxide ash left from these quantities has been found by experience to be most suitable; if the mass is greater than this, the light-emission is diminished without a compensating gain in strength; if it is less, the light-emission is indeed greater, but the mantle becomes too fragile.