CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC, 295 

 Both metals are infusible, and both under the action of heat form 



anhydrous, sparingly soluble tetratungstate, Na 2 WO 4 ,3WO 5) which, when heated at 120 

 in a closed tube with water, passes into an easily soluble metatungstate. It may there- 

 fore be said that the metatungstates are hydrated compounds. On boiling a solution of 

 the above-mentioned salts of sodium with the yellow hydrate of tungstic acid they give 

 a solution of metatungstate, which is the hydrated tetratungstate. Its crystals contain 

 NaoW 4 O 13 ,10H 2 O. After the hydrae of tungstic acid (obtained from the ordinary tung- 

 states by precipitation with an acid) has stood a long time in contact with a solution (hot 

 or cold) of sodium tungstate, it gives a solution which is not precipitated by hydrochloric 

 acid ; this must be filtered and evaporated over sulphuric acid in a desiccator (it is de- 

 composed by boiling). It first forms a very dense solution (aluminium floats m it) of 

 sp. gr. S'O, and octahedral crystals of sodium metat'ungstate, Na 2 W 4 O 1 3,10H 2 O, sp. gr. 

 3'85, then separate. It effloresces and loses water, and at 100 only two out of the 

 ten equivalents of water remain, but the properties of the salt remain unaltered. If the 

 salt be deprived of water by further heating, it becomes insoluble. At the ordinary 

 temperature one part of water dissolves ten parts of the metatungstate. The other 

 metatungstates are easily obtained from this salt. Thus a strong and hot solution, 

 mixed with a like solution of barium chloride, gives on cooling crystals of barium meta- 

 tungstate, BaW 4 O 13 ,9H 2 O. These crystals are dissolved without change in water con- 

 taining hydrochloric acid, and also in hot water, but they are partially decomposed by 

 cold water, with the formation of a solution of metatungstic acid and of the normal 

 barium salt BaWO 4 . 



In order to explain the difference in the properties of the salts of tungstic acid, we 

 may add that a mixture of a solution of tungstic acid with a.solution of silicic acid does 

 not coagulate when heated, although the silicic acid alone would do so ; this is due to 

 the formation of a silicotungstic acid, discovered by Marignac, which presents a fresh 

 example of a complex acid. A solution of a tungstate dissolves gelatinous silica, just as 

 it does gelatinous tungstic acid, and when evaporated deposits a crystalline salt of 

 silicotungstic acid. This solution is not precipitated either by acids (a clear analogy to 

 the metatungstates) or by sulphuretted hydrogen, and corresponds with a series of salts. 

 These salts contain one equivalent .of silica and 8 equivalents of hydrogen or metals, in 

 the same form as in salts, to 12 or 10 equivalents of tungstic anhydride ; for example, 

 the crystalline potassium salt has the composition K 8 Wi 2 SiO 42 ,14H.jO = 4K 2 O,12WO 3 , 

 SiOjjlilLjO. Acid salts are also known in which half of the metal is replaced by 

 hydrogen. The complexity of the composition of such complex acids (for example, of 

 the phosphomolybdic acid) involuntarily leads to .the idea of polymerisation, which we 

 were obliged to recognise for silica, lead oxide, and other compounds. This polymerisa- 

 tion, it seems to me, may be understood thus : a hydrate A (for example, tungstic acid) 

 is capable of combining with a hydrate B (for example, silica or phosphoric acid, with 

 or without the disengagement of water), and by reason of this faculty it is capable of 

 polymerisation that is, A combines with A combines with itself just as aldehyde, 

 GjH 4 O, or the cyanogen compounds are able to combine with hydrogen, oxygen, &c., 

 and are liable to polymerisation. On this view the molecule of tungstic acid is probably 

 much more complex than we represent it , this agrees with the easy volatility of such 

 compounds a* the chloranhydrides, CrO a Cl 2 , MoO 2 Clj, the analogues of the volatile 

 eulphuryl chlcride, SO C1 2 , and with the non-volatility, or difficult volatility, of chromic . 

 and molybdic annydrides, the analogues of the volatile sulphuric anhydride. Such a 

 view also finds a certain confirmation in the researches made by Graham on the colloidal 

 state of tungstic acid, because colloidal properties only appertain to compounds of a very 

 complex composition. The observations made by Graham on the colloidal state of 

 tungstic and molybdic acids introduced much new matter into the history of these sub'- 

 stances. When sodium tungstate, mixed in a dilute solution with an equivalent quantity 

 of dilute hydrochloric acid, is placed in a dialyser, hydrochloric acid and sodium chloride 

 pass through the membrane, and a solution of tungstic acid remains in the dialyser. 

 Out of 100 parts of tucgstic acid about 80 parts remain in the dialyser. The solution 

 *B 



