302 Schaller — Chemical Composition of Molybdic Ocker. 



molybdate to an excess of a solution of a ferric salt, no precipi- 

 tate is formed, but on reversing the process and adding the 

 ferric salt to the ammonium molybdate, a voluminous yellow 

 precipitate appears. A precipitate thus prepared was air-dried 

 for about a week and analyzed with the following result : 







Eatio 



Fe 2 O s 



15-9 



1-0 



Mo0 3 



61-6 



4-3 



H„0 (by diff.) 



22-5 



12'5 



100-0 



These results are between those of the two salts above quoted. 

 It was found, however, that a large amount of free molybdic 

 acid contaminated the material and the impossibility of air- 

 drying the non-crystalline mass sufficiently accounts for the 

 high water content. It is therefore believed that neither of the 

 two salts above mentioned and described in chemical diction- 

 aries has any existence, but that they are mixtures of a salt of 

 the formula Fe 2 3 .3Mo0 3 -f n(3II 2 0) with molybdic acid and 

 water. 



A preliminary experiment of heating precipitated ferric 

 hydroxide and molybdic acid in the correct molecular propor- 

 tions with an excess of water in a glass bomb failed to give any 

 result. After heating to about 150° for several hours, there 

 was no indication of any reaction and on higher heating the 

 bomb exploded. 



An attempt was next made to crystallize the yellow amor- 

 phous precipitate. The precipitate was heated with water in 

 a glass bomb up to 180°-200° for several days, and when 

 examined, was found to consist of a mass of fine yellow crystals 

 and a greenish amorphous mass. By shaking up the tube, the 

 green part settled very quickly, while the minute yellow 

 crystals settled so slowly that a nearly perfect separation 

 of the two products could be made. The yellow crystalline 

 part was examined under the microscope and found to consist 

 of three products, all crystallized. The most abundant salt 

 occurs in minute pale yellow tablets of quadratic outline 

 and sometimes with an octagonal shape suggesting combina- 

 tions of the cube and octahedron, and which seem to be 

 isotropic. They were too small to test for interference 

 figures. The second most abundant salt occurs in pale yellow 

 fibrous prisms and also in radiated fan-shaped masses that show 

 the characteristic absorption of the natural mineral, and are 

 probably to be identified with it. The third salt also occurs 

 in prisms which, however, are not fibrous, do not show any 

 absorption, and seem to be colorless. They may be molybdic 



