of Molybdenum and some of its Compounds. 525 



very astringent taste. Both water and alcohol dissolve it 

 readily, the latter with a yellow colour, which on evaporation 

 changes to blue, and leaves a brown opake residue, which 

 dissolves in water with a blue colour. When an excess of 

 molybdic acid is digested with phosphoric acid, the latter is 

 precipitated and forms with the molybdic acid a lemon-co- 

 loured basic salt which is insoluble in water." 



These observations are quoted by Gmelin*, who has added 

 to them the following: — " When a very little phosphoric acid is 

 added to a concentrated solution of the molybdate of ammonia, 

 and then large quantities of muriatic, sulphuric, or nitric acid, 

 or first one of these acids and then a little phosphoric acid, 

 a lemon- coloured crystalline powder falls, the more quickly 

 the more concentrated the liquid ; this powder contains besides 

 molybdic acid some ammonia, but none, or extremely little, 

 phosphoric acid ; it forms with potash a colourless solution with 

 evolution of ammonia, from which it is again precipitated by 

 muriatic acid by long boiling; but if the solution in potash is 

 evaporated to dryness, and the residue dissolved in water, mu- 

 riatic acid then merely gives it a yellow colour, but muriatic 

 acid and chloride of ammonium precipitate a yellow powder. 

 The yellow powder gives with hot sulphuric acid a colourless 

 solution which is not precipitated by water. It is but very 

 sparingly soluble in cold water, dilute sulphuric, hydrochloric, 

 or nitric acid, it dissolves slightly in boiling water without 

 colour." 



When molybdic acid, obtained by calcining a pure molyb- 

 date of ammonia, is mixed with an excess of phosphoric acid, 

 scarcely a trace is dissolved at the ordinary temperature ; but 

 on heating the mixture, a perfect and colourless solution soon 

 results, which forms, when quickly evaporated, a colourless vi- 

 treous non-crystalline mass. But if some water is added to this, 

 or it is allowed to absorb moisture fromkhe air, it acquires a 

 beautiful blue colour, which however again disappears on 

 evaporation. If, on the contrary, an excess of molybdic acid 

 is employed, and gradually some drops of phosphoric and 

 nitric acid added to it (we employ nitric acid to be sure of 

 having all the molybdenum as molybdic acid), on the applica- 

 tion of heat the colourof the molybdic acid first becomes lemon- 

 yellow, then gradually the solution acquires also a yellow co- 

 lour ; a portion of the molybdic acid remains unaltered, whilst 

 another portion is converted into a yellow powder. When 

 nothing further is dissolved the solution is filtered ; on evapo- 

 rating it to dryness to expel the nitric acid, a yellowish-white 

 amorphous mass is obtained, which dissolves in water, and also 

 * llandbuch dcr Chenvc, 1844, vol. ii. p. 501, 51 1. 



