OP ARTS AND SCIENCES. 241 



refer to them under the name of vanaclico-vanadio-molybdates. The 

 presence even of a small quantity of vanadic dioxide may usually be 

 detected by a peculiar greenish shade of color, easily recognized when 

 once seen. This tint disappears when the solution is heated for a 

 short time with bromine water or nitric acid, which readily oxidize 

 the lower oxides of vanadium. 



Vanadio-molybdates are also formed when solutions of alkaline va- 

 nadates and raolybdates are heated together, and more readily when an 

 excess of acid is present. The mixed solution then assumes at once 

 a deep orange-red color. 



The same salts are also formed when molybdic oxide, M0O3, is 

 boiled with a solution of an alkaline vanadate or meta-vanadate. 

 Ammonic meta-vanadate, VOgNH^, may now be had in commerce in a 

 state of very considerable purity, and has served as the starting point 

 for much of my work. A solution of this salt readily dissolves molybdic 

 oxide, forming one or more vanadio-molybdates, only in this case there 

 is a tendency to a kind of supersaturation, a large excess of molybdic 

 oxide being dissolved and separating from the liquid on cooling. 



Finally, vanadio-molybdates may be formed by the decomposition 

 of phospho-vanadio-molybdateSj a class of triple compounds also to be 

 described. 



The vanadio-molybdates of the alkaline metals are in general read- 

 ily soluble in water. They are highly crystalline, have a yellow or 

 greenish yellow, sometimes orange-yellow or orange-red color, and 

 give in many cases crystalline precipitates with salts of the alkaline 

 earths, of silver, and of thallium. They are not decomposed by boiling 

 with excess of acid, and are usually very stable, except in the pres- 

 ence of reducing agents. 



Analytical Methods. — The quantitative separation of molybdenum 

 from vanadium presented a new problem in analytical chemistry, the 

 solution of which cost much time and labor. It may be effected in 

 two different ways. When the vanadium is wholly or jiartially in the 

 form of vanadic dioxide, VO, , it is to be oxidized by heating the con- 

 centrated solution with nitric acid. An alkaline carbonate is then to 

 be added until the solution is nearly neutral, when the vanadic and 

 molybdic oxides may be precipitated together by means of mercurous 

 nitrate and mercuric oxide, precisely as in the estimation of tungstic 

 oxide already described. The precipitate of mercurous vanadate and 

 molybdate and mercuric oxide is to be filtered upon a paper filter. 

 After drying, the filter and contents may be heated together in a plati- 

 num crucil)le, with free access of air. When the process is carefully 



VOL. XVIII. (x. s. X.) 16 



