181 MOLYBDENITE. 



ammonium. The sulphide of lead which is thus 

 formed, is filtered off from the dissolved molybdate, 

 washed with dilute sulphide of ammonium, dried at 

 100, and weighed. From the solution, the sulphide 

 of molybdenum is precipitated by dilute nitric acid; 

 collected upon a filter, dried at 100, washed, dried, 

 and weighed. A weighed portion of it is then intro- 

 duced into a bulb-tube (Fig. 19), and heated in a 

 stream of hydrogen until it loses no more sulphur. 

 From the weight of the residual MoS 2 , calculated for 

 the total amount of the precipitate, that of the molyb- 

 dic acid is ascertained. 



99. MOLYBDENITE. 



MoS 2 . 



The mineral in small fragments is heated in a tube 

 of hard glass, through which a very slow stream of 

 dry air is passed, forming sulphurous acid and molybdic 

 acid, the latter subliming in colorless needle-shaped 

 crystals. 



If a large quantity of the material is to be used, it 

 is better to mix the finely powdered mineral with an 

 equal volume of pure sand, and roast it at a red heat, 

 frequently stirring it in the inclined crucible, until the 

 smell of sulphurous acid is no longer perceptible, and 

 the mass has become yellow. 



The molybdic acid which has been formed is ex- 

 tracted from the yellow mass thus produced, by diges- 

 tion with dilute ammonia. The residue, should it still 

 contain sulphide of molybdenum, is then again 

 roasted. 



The filtered solution is mixed with one or two drops 

 of sulphide of ammonium, to separate the copper, the 

 precipitate filtered off, the solution evaporated to dry- 

 ness, the salt again dissolved in dilute ammonia, the 



