CHROMIUM. MOLYBDENUM, TUNGSTEN, URANIUM, ETC, 293 

 and W) reduces molybdic, and tungstic anhydride at a red heat ; and 



on the fact that the atoms cf hydrogen in phosphoric acid are of a very different 

 character, as we saw above. Those atoms of hydrogen whicli are replaced with difficulty 

 by ammonium, sodium, &c., are probably easily replaced by feebly energetic acid 

 groups that is, the formation of particular complex substam.-es may be expected to 

 take place nt the expense of these atoms of the hydrogen of phosphoric acid and of 

 certai'n feeble metallic acids ; and these substances will still be acids, because the 

 hydrogen of the phosphoric acids and metallic acids, which is easily replaced by metals, 

 is not removed by Iheir mutual combination, but remains in the resultant compound. 

 Such a conclusion is verified in the phosphomolybdic acids obtained (1888) by Debray 

 If a solution of ammonium molybdate be acidified, and a small amount of a solution (it 

 may be acid) containing orthophosphoric acid or its salts be added to it (so that there are 

 at least 40 parts of molybdic acid present to 1 part of phosphoric acid), then after a period 

 of twenty-four hours the whole of the phosphoric acid is separated as a yellow precipitate, 

 containing, however, not more than 8 to 4 p.c. of phosphoric anhydride, about 8 p.c. of 

 ammonia, about 90 p.c. of molybdic anhydride, and about 4 p.c. of water. The formation 

 Of this precipitate is so distinct and so Complete that this method is employed for the dis- 

 covery and separation of the smallest quantities of phosphoric acid. Phosphoric acid was 

 found to be present in the majority of rocks by this means. The precipitate is soluble 

 in ammonia and its salts, in alkalis and phosphates, but is perfectly insoluble in nitric, 

 sulphuric, and hydrochloric acids in the presence of ammonium molybdate. The compo- 

 sition of the precipitate appears to vary under the conditions of its precipitation, but its 

 nature became clear when the acid corresponding with it was obtained. If the above- 

 described yellow precipitate be- boiled in aqua regia, the ammonia is destroyed, and 

 an acid is obtained in solution, which, when evaporated in the air, crystallises out in yellow 

 oblique prisms of approximately the composition P 2 O 5 ,20MoO3,26H 2 O Such an unusual 

 proportion of component parts is explained by the above-mentioned considerations. We 

 saw above that molybdic acid easily gives salts R. 2 OwMoO 5 mH 2 O, which we may imagine 

 to correspond to a hydrate MoO 2 (HO).jMoO5mH 2 O. And suppose that such a hydrate 

 reacts on orthophosphoric' acid, forming water and compounds of the composition 

 MoO 2 (HPO 4 )7iMoO 3 ?/iH 2 or MoO 5 (H 2 PO 4 ) j 7iMoO 5 >rcHoO ; this is actually the composi- 

 tion of phosphomolybdic acid. Probably it contains a portion of the hydrogen replaceable 

 by metals of both the acids H 5 PO 4 and of H 2 Mo0.i. The crystalline acid above is 

 probably H-,MoPO 7 ,9Mo0 3 ,12H.jO. This acid is really tribasic, because its aqueous 

 Solution precipitates salts of potassium, ammonium, rubidium (but not lithium and 

 sodium) from acid solutions, and gives a yellow precipitate of the composition 

 R 3 MoPO 7 ,9MoO 3 ,8H 2 O, where R = NH 4 . Besides these, salts of another composition 

 xnay be obtained, as would be expected from the preceding. These salts are only stable 

 in acid solutions (which is naturally due to their containing an excess of acid oxides), 

 whilst under the action of alkalis they give colourless phosphomolybdates of the compo- 

 sition R3MoPO r) ,MoO 2 ,8H 2 0. The corresponding salts of potassium, silver, ammonium, 

 are easily soluble in water and crystalline. 



Phosphomolybdic acid is an example of the complex inorganic acids first obtained 

 by Marignac and afterwards generalised and studied in detail by Gibbs. We shall 

 afterwards meet with several examples of such acids, and we will now turn attention to 

 the fact that they are usually formed by wtfak polybasic acids (boric, silicic, molybdic, 

 &c.), and in certain respects resemble the cobaltic and such similar complex compounds, 

 with which we shall become acquainted in the following chapter. As an example we will 

 here mention certain complex compounds containing molybdic and tungstic acids, aathey 

 will illustrate the possibility of a considerable complexity in the composition of salts. 

 The action of ammonium molybdate upon a dilute solution of purpureo-cobaltic salts (see 

 Chapter XXII.) acidulated with acetic acid gives a salt which after drying at 100 has the 

 composition Co.jO3lONH 3 7MoO33H 2 O. After ignition this salt leaves a residue having the 

 composition 2CoO?MoO s . An analogous compound is also obtained for tungstic acid, having 

 'ibe composition Co.jO 5 10NH 5 10WO 3 9H,,0. In this case after ignition there remains a salt 



