fg-l Method of extracting Molyhdic Acid' 



alwavs contains in greater or less quantity. This proces??, 

 which was considered as the only one proper for obtaining 

 pure molybdic acid, besides its not answering the proposed 

 end, is too expensive to be followed, and therefore another 

 more (Economical and more proper for obtaining an acid 

 free from foreign mixture is desirable to be substituted ia 

 its stead. 



The third process consists in distilling the sulphurated 

 molybdena with arsenic acid, and in expelling by volatiliza- 

 tion the oxide and sulphurct of arsenic which are formed. 

 This method also is so expensive and so dangerous that we 

 shall not enter into aily further discussion of it. 



The fovu-th process, which belongs also to Scheele, is 

 that which furnishes the acid in the speediest manner; but 

 it does not furnish it pure. It consists in causing the mo- 

 Ivbdena to deflagrate with nitrate of potash in proportions 

 respecting which authors vary. Scheele * took four parts 

 of iiitrate to one of sulphuret of molybdena; Lampadiusf 

 recommends six parts of nitrate to one of molybdena, and 

 Fourcroy % three parts of nitrate to one of molybdena. 



The sulphur and the. metal are here converted into acids, 

 and in that state combine with the potash. A portion of 

 the nitrate remains undecomposed, and there is formed at 

 the same time deoxygenated nitrate : the remaining mass is 

 dissolved in warm water ; and the molybdatc of potash, still 

 mixed with its other salts, is decomposed (or this may be 

 done after it has been separated by crystallization) by means 

 of the nitric, muriatic, or sulphuric acids. The molybdic 

 acid, when the solution is not too dilute, separates under 

 the concrete form : this precipitate is not pure molybdic 

 acid, but, as Scheele before announced, molybdic acid hold- 

 ing; potash, or, according to Fourcroy, acidulous molybdate 

 of that alkali. 



This acidulous molybdate is much more soluble in water 

 than Dure molybdic acid, which for its solution requires 500 

 parts of that licpaid, whereas acidulous molybdate dissolves 

 in three or four parts of water. The latter fuses in the fire 

 sooner than the acid, and more readily corrodes the cruci- 

 ble. After cooling, it is of a beautiful yellow colour : the 

 acid, on the contrary, exhibits a radiant mass of a whitish 

 gray colour, having a slight metallic brilliancy. The acidu- 

 lous molybdatc does not suffer itself to be sublimated ; 

 whereas the pure acid is vaporized by heat, and deposits it- 



* Opuscu'a Physica ct Cliemica, torn. i. p. aoi. 

 t Handbuch der Chemischcii Analyse, p. 3;5. 

 1 Svsteuie des Connoissances Chimiqucs. 



self, 



