WULFENITE. 



183 



constantly stirred. The molybdic acid is thus sepa- 

 rated as a white precipitate; when the greater part of 

 the sulphuric acid has been expelled, the mixture is 

 diluted with water, the molybdic acid filtered off' and 

 well washed, water containing nitric acid being used 

 towards the end. Some more molybdic acid may be 

 obtained by evaporating the filtrate and washings. It 

 is free from phosphoric acid. 



If it contain any phosphoric acid, its ammoniacal 

 solution, when acidified with nitric acid, and heated, 

 becomes yellow and deposits a yellow powder. 



The molybdic acid may be obtained from molybdate 

 of ammonia by gradually heating the salt, with free 

 access of air. 



It is always obtained in a perfectly pure state by 

 sublimation, for which purpose it is heated in a plati- 

 num crucible, which is covered with a platinum cap- 

 sule kept full of water. 



For quantitative analysis, the pure crystallized 

 mineral is finely powdered, completely decomposed by 



Fig. 19. 



digestion with nitric acid, the mixture neutralized with 

 ammonia, and digested with an excess of sulphide of 



