OF ARTS AND SCIENCES. 257 



PHOSrHO-VANADIO-TUNGSTATES. 



These compounds bear a general resemblance to the phospho-vanadio- 

 molybdates already described, and may be prepared by similar processes. 

 They have usually a more or less deep orange or orange-red color, and 

 very often exhibit a peculiar and highly characteristic aurora-red tint. 

 They are crystalline and well defined, and many salts of the different 

 series are very beautiful. 



Analytical Methods. — In all these salts it will, as in similar cases, 

 be found most advantasfeous to determine the three acid-formiun: oxides 

 by means of mercurous nitrate and mercuric oxide in the manner 

 already pointed out. The estimation of phosphoric oxide may usually 

 be made with a fair approximation to precision by direct precipitation 

 with magnesia-mixture in the ammoniacal solution, redissolving the 

 ammonio-magnesic j^hosphate, and precipitating a second lime. In 

 some cases I have found it better first to reduce the vanadic pentoxide 

 to vanadic dioxide by boiling with strong chlorhydric acid, evaporating 

 to dryness, and redissolving with the addition of a little tartaric and 

 chlorhydric acids. A perfectly satisfactory method of estimating phos- 

 phoric oxide in these compounds remains to be discovered. Vanadic 

 pentoxide may be estimated with accuracy by the methods already 

 pointed out for the analysis of the vanadio-tungstates ; that is, by boil- 

 ing with strong chlorhydric acid, collecting the chlorine evolved in a 

 solution of potassic iodide, and determining the free iodine by titrition 

 with sodic hyposulphite. A solution of potassic hypermanganate does 

 not always oxidize vanadic dioxide in presence of tungstic, phosphoric, 

 and free sulphuric acid, or only with great diflaculty, so that tlie 

 method is not generally applicable for the analysis of this class of 

 compounds. Nitric acid, however, readily oxidizes the dioxide under 

 the same circumstances. 



60 : 3 : 1 Series. — When the white insoluble phospho-tungstates of 

 ammonium containing twenty-two or twenty-four molecules of tungstic 

 to one of phosphoric oxide are dissolved in ammonia water, and a solu- 

 tion of ammonic meta- vanadate is added, no precipitate is formed ; but 

 if a small excess of chlorhydric acid is present in the boiling liquid, a 

 fine lemon-yellow crystalline precipitate soon forms, which is to be well 

 washed with a cold dilute solution of ammonic nitrate, as otherwise 

 the salt passes readily through the filter, giving a milky liquid. The 

 final washing may be made with a very dilute solution of the nitrate. 

 After drying on woollen paper, the salt closely resembles tungstic oxide 

 in color. It is almost insoluble in cold, and but slightly soluble in hot 



VOL. XVIII. (n. s. X.) 17 



