292 PRINCIPLES OF CHEMISTRY 



Hydrogen (which does not directly form compounds with Cr, Mo, 



4'4; whilst on further heating in open vessels or in a stream of air this anhydride 

 sublimes in pearly scales this enables it to be obtained in a tolerably pure state. Water 

 dissolves it in small quantities namely, 1 part requires 600 parts of water for its solution. 

 The hydrates of molybdic anhydride are soluble also in acids (a hydrate, H.^MoC^, is 

 obtained from the nitric acid solution of ihe ammonium salt), which forms one of their 

 distinctions from the tungstic acids. But after ignition molybdic anhydride is insoluble 

 in acids, like tungstic anhydride ; alkalis dissolve this anhydride, easily forming molybdates. 

 Potassium bitartrafce dissolves the anhydride with the aid of heat. None of the acida 

 yet considered by us form so many different salts with one and the name base (alkali) as 

 molybdic and tungstic acids. The composition of these salts, and their properties also, 

 vary considerably. The most important discovery in this respect was made by Margue- 

 rite and Laurent, who showed that the salts which contain a large proportion of tungstic 

 acid are 1 easily soluble in water, and ascribed this property to the fact that tungstic acid 

 may be obtained in several states. The common tungstates, obtained with an excess of 

 alkali, have an alkaline reaction, and on the addition of sulphuric or hydrochloric acid 

 first deposit an acid salt and then a hydrate of tungstic acid, which is insoluble both in 

 water and acids ; but if instead of sulphuric or hydrochloric acids, we add acetic or phos- 

 phoric acid, or if the tungstate be saturated with a fresh quantity of tungstic acid, which, 

 may be done by boiling the solution of the alkali salt with the precipitated tungstic acid, 

 a solution is obtained which, on the addition of sulphuric or a similar acid, does not- 

 give a precipitate of tungstic acid at the ordinary or at higher temperatures. Thesolutioa 

 then contains peculiar salts of tungstic acid, and if there be an excess of acid it also- 

 contains tungstic acid itself ; Laurent, Riche, and others called it tnetatungstic acid, and 

 it is still known by this name. Those salts which with acids immediately give the in, 

 soluble tungstic acid have the composition R 2 W0 4 , RHWO 4 , whilst those which give 

 the soluble metatungstic acid contain a far greater proportion of the acid elements.- 

 Scheibler obtained the (soluble) metatungstic acid itself by treating the soluble barium 

 (meta) tetratungstate, BaO,4WO 3 , with sulphuric acid. Subsequent research showed the- 

 existence of a similar phenomenon for molybdic acid. There is no doubt that this is a 

 case of colloidal modifications. 



Many chemist's have worked on the various salts formed by molybdic and tungstic 

 acids. The tungstates have been investigated by Marguerite, Laurent, Marignac,. x 

 Riche, Scheibler, Ahthon, and others. The molybdates were partially studied by the 

 same chemists, but chiefly by Struve and Svanberg, Delafoutaine, and others. It appears- 

 that for a given amount of base the salts contain one to eight equivalents of molybdic or 

 tungstic anhydride ; i.e. if the base have the composition RO, then the highest proportion 

 of base will be contained by the salts of the composition ROWO 3 or ROMoO 3 that is, by 

 those salts which correspond with the normal acids H 2 WO 4 and H 2 MoO 4 , of the same nature- 

 as sulphuric acid ; but there also exist salts of the composition RO,2WO 3 , KO,3WO 3 

 RO,8WOs. The water contained in the composition of many of the acid salts is often 

 not taken into account in the above. The properties of the salts holding different pro- 

 portions of acids vary considerably, but one salt may be converted into another by the 

 addition of .acid or base with great facility, and the greater the proportion of the elements 

 of the acid in a salt, the more stable, within a certain limit, is its solution and the salt- 

 itself. 



The most common ammonium molybdate has the composition (NH 4 HO) 6 ,H 2 O,7Mopj 

 (or, according to Marignac and others, NH 4 HMoO 4 ), and is prepared by evaporating an 

 ammoniacal solution of molybdic acid. It is used in the laboratory for precipitating 

 phosphoric acid, and is purified for this purpose by mixing its solution with a small 

 quantity of magnesium nitrate, in order to precipitate any phosphoric acid present, filter- 

 ing, and then adding nitric acid and evaporating to dryness. A pure ammonium molyb- 

 date free from phosphoric acid may then be extracted from the residue. 



Phosphoric acid forms insoluble compounds with the oxides of uranium and iron, 

 tin, bismuth, &c., having feeble basic and even acid properties. This perhaps depends. 



