294 PRINCIPLES OF CHEMISTRY 



this forms the means of obtcaining metallic molybdenum and tungsten, 



of the composition CoO5WO 5 (Carnbt, 1889). Professor Kournakoff, by treating a solution of 

 potassium and sodium molybdates, containing a certain amount of suboxide of cobalt, with 

 bromine obtained salts having the composition : 8K 2 OCo. 2 O 3 12MoO 3 20H a O (light green) 

 and SKjOCo^OjlOMo^lOBLjO (dark green). Pe'chard (1893) obtained saltsof the four complex 

 phosphotungstic acids by evaporating equivalent mixtures of solutions of phosphoric acid 

 and metatungstic acid (see further on): phosphotrimetatungstic acid P 3 O5l2WO 3 48H 2 O r 

 phosphotetrameiatungstic acid P 2 05l6WO 5 69H 2 O, phosphopentametatungstic acid 

 P 2 O520WO 3 H 2 O, and phosphphexametatungstic acid P a O 5 24WO 5 59H 2 O. Kehrmann 

 and Frankel described still more complex salts, such as: 8Ag 2 O4BaOP 2 5 22WO 3 H, 2 O, 

 6BaO'2K 2 OP 2 O 3 22WO 3 48H 2 O. Analogous double salts with 22WO 5 were also obtained 

 with KSr, KHg, BaHe, and NH 4 Pb. Kehrmann (1892) considers the possibility of 

 obtaining an unlimited number of such salts to be a general characteristic of such com- 

 pounds. Mahom and Friedheim (1892) obtained compounds of similar complexity for 

 raolybdic and arsenic acids. 



For tungstic acid there are known : (1) Normal salts for example, K a WO 4 ; (2) the 

 so-called acid salts have a composition like 8K 2 O,7WO 3) 6H 2 O or K 6 H 8 (WO 4 ) 7 ,2H 2 O ; (3> 

 the tritungstates like Na 3 O,3WO 3) 3H5p = Na J H 4 (WO 4 ) 3 ,H 2 O. All these three classes of 

 salts are soluble in water, but are precipitated by barium chloride, and with acids in solu- 

 tion give an insoluble hydfate of tungstic acid ; whilst those salts which are enumerated 

 below do not give >a precipitate either with acids or with the salts of the heavy metals, be- 

 cause they form soluble salts even with barium and lead. They are generally called meta- 

 tungstates. They all contain water and a larger proportion of acid elements than the 

 preceding salts ; (4) the tetratungstates, like Na,iO,4WO 3 ,10H 2 O and BaO,4WO 3 ,9H 2 O for 

 example ; (5) the octatungstates for example, Na-jO.SW Oj^H^O. Since the metatung- 

 states lose so much water at 100 that they leave salts whose composition corresponds 

 with an acid, 8H 2 O,4WO 3 that is, H 6 W 4 O 15 whilst in the meta salts only 2 hydrogens 

 ore replaced by metals, it is assumed, although without much ground, that these salts- 

 contain a particular soluble metatungstic acid of the composition H 6 W 4 Oi 5 . 



N As an example we will give a short description of the sodium salts. The normal 

 Bait, Na 2 WO 4 , is obtained by heating a strong solution of sodium carbonate with tungstic- 

 ftcid to a temperature of 80 ; if the solution be filtered hot, it crystallises in rhombic 

 tabular crystals, having the composition Na 2 WO 4 ,2H 2 O, which remain unchanged in the 

 air and are easily soluble in water. When this salt is fused with a fresh quantity of 

 tuugstic acid, it gives a ditungstate, which is soluble in water and separates from ita 

 solution in crystals containing water. The same salt is obtained by carefully adding 

 hydrochloric acid to the solution of the normal salt so long as a precipitate does not 

 appear, and the liquid still has an alkaline reaction. This salt, was first supposed to 

 have the composition Na 2 W 2 O 7 ,4H 2 O, but it has since been found to contain (at 100) 

 NaeW 7 O 24 ,16H 2 O that is, it corresponds with the similar salt of molybdic acid. 



(If this salt be heated to a red heat in a-stream of hydrogen, it loses a portion of its 

 oxygen, acquires a metallic lustre, and" turns a golden yellow colour, and, after being- 

 treated with water, alkali, and acid, leaves golden yellow leaflets and cubes which are 

 very like gold. This very remarkable substance,. discovered by W6hler, has, according 1 

 to Malaguti's analysis, the composition Na 2 ! W 5 O 9 ; that, isj it, as it were, contains a- 

 double tungstate of tungsten oxide, WO 2 , and of sodium, Na. 2 WO 4 ;WO 2 WO 3 r ' The 

 decomposition of the fused sodium salt is best effected by finely-divided tin. This sub- 

 stance has a sp. gr. 6'6; it conducts electricity, like metals, and like them has a metallic 

 lustre. When brought into contact with zinc and sulphuric acid it disengages hydrogen, 

 and it becomes covered with a coating of copper in a solution of copper sulphate in the 

 presence of zinc that is, notwithstanding its complex composition it presents to a 

 certain extent the appearance and reactions of the metals. It is not acted on by aqua- 

 rcgia or alkaline solutions, but it is oxidised when ignited in air.) 



The ditungstate mentioned above, deprived of water (having undergone a modifica- 

 tion similar to that of metaphosphoric acid), after being treated with water, leaves anr 



