of Molybdenum and some of its Compounds. 531 



in the analysis of any felspar, dissolve it in an acid, then add 

 some pure molybdate of ammonia, and afterwards an excess 

 of an acid, a copious yellow precipitate instantly falls. If we 

 attempt to demonstrate the presence of phosphoric acid in the 

 same alumina by a solution of magnesia (preventing the preci- 

 pitation of the alumina by ammonia by the addition of tartaric 

 acid), the reaction only occurs after several hours, and is then 

 always very imperfect. 



Phosphoric acid has so great a tendency to accompany mo- 

 lybdic acid in all its combinations, that we are unable to direct 

 any other plan for procuring pure molybdic acid than by 

 preparing an artificial sulphuret in the dry or humid way. 

 In the decomposition of the potash double salt (obtained by 

 acting with nitric acid upon a solution of molybdic acid in 

 potash) by water, intotrimolybdate of potash, which separates, 

 and a neutral salt which remains in solution, traces of phos- 

 phoric acid accompany the insoluble compound. The pre- 

 sence of the phosphoric acid in this salt is best proved by 

 converting it into the silver salt. On decomposing this 

 salt with muriatic acid, separating the chloride of silver and 

 evaporating the filtered solution, with the addition of a few 

 drops of ammonia, a yellow precipitate of the ammonia salt is 

 always formed : its amount is very small, and it has no in- 

 fluence upon the analysis; but it is quite sufficient to show the 

 presence of the phosphoric acid. 



III. Determination of the Atomic Weight of Molybdenum. 



We shall here describe a large number of experiments 

 which we have made in order to determine accurately the 

 atomic weight of this metal. Although most of these expe- 

 riments have not led to the desired result, they will never- 

 theless be of scientific interest. If, from their not being so 

 accurate as we had reason to expect, they did not assist in 

 fixing the atomic weight, — and exhibited in several instances 

 differences from the theories previously admitted, — they never- 

 theless remain quantitative results, which show in what manner 

 various compounds of molybdenum behave towards different 

 reagents. 



The atomic weight of molybdenum was ascertained by Ber- 

 zelius in 1818, by precipitating a certain quantity of nitrate 

 of lead with neutral molybdate of ammonia, and calculating 

 the atomic weight from the quantity of molybdate of lead 

 thus obtained. Berzelius, however, says of this determination, 

 " that it is far from being trustworthy, and the number found 

 is far more, erroneous than the atomic weight of chromium 

 ascertained in the same manner. The atomic weight of mo- 



