of Molybdenum and some of its Compounds. 415 



phoric acid and excess of sulphur are removed from the sul- 

 phuret of molybdenum, which gradually acquires a pure black 

 colour, and when pressed with a glass rod immediately ex- 

 hibits the metallic lustre peculiar to the sulphuret. The 

 sulphuret is now treated with water containing muriatic acid, 

 and washed upon the filter with hot water. This sulphuret 

 of molybdenum is pure; and to prepare molybdic acid from 

 it, it may either be roasted or treated with nitric acid ; the 

 oxidation by nitric acid is readily effected, as the sulphuret is 

 in a very finely divided state. 



In most cases this circuitous process is not requisite, espe- 

 cially when the object is to prepare combinations with other 

 bases ; a more simple plan may then be adopted, as will be 

 stated under the different compounds. 



Molybdic acid is easily tested as to its freedom from fixed 

 impurities ; a portion exposed to a strong red heat in a cru- 

 cible with free access of air, should be entirely volatilized. 



Salts of Potash. 



As stated in the introduction, the potash salts have in part 

 been briefly described by various chemists, but many of them 

 have only been used for the preparation of some of the in- 

 soluble compounds. All the statements respecting them are 

 very imperfect, and founded upon mere test experiments with- 

 out any analysis. The following salts have been examined 

 by us. 



Neutral Molybdate of Potash. — The formula for the salt 

 dried over sulphuric acid is KaO, Mo0 3 + ^HO. We have 

 prepared this salt in different ways. 



A. Either of the ammonia salts is treated with an excess of 

 carbonate of potash, the solution evaporated by heat to the 

 consistence of a syrup, when the salt gradually separates in the 

 cold. The salt however cannot be thus obtained free from 

 carbonate of potash, as will be shown by the analysis. 



B. Caustic potash is dissolved in alcohol of 0*815 spec. grav. 

 and small portions of moist trimolybdate of potash added, 

 and the whole shaken in a well-corked bottle. The neutral 

 salt separates as an oily mass at the bottom of the bottle ; the 

 supernatant alcoholic solution of potash is removed with a 

 pipette, the oily mass washed with alcohol, then poured into a 

 dish and left to crystallize over caustic lime and sulphuric acid. 



The neutral molybdate of potash crystallizes in four-sided 

 prisms having two truncations on the smaller lateral faces of 

 the prism. It is readily soluble in water after ignition. When 

 heated it parts with its water and falls to a white powder; it 

 requires the highest temperature which can be produced with 

 an Argand lamp to fuse it; it then flows readily, but soli- 



