OP ARTS AND SCIENCES. 81 



perfectly constant weight. The difference between the percentage 

 of arsenic oxide, As^jOg, and the sum of the percentages of the arsenic 

 and molybdic oxides, gives the percentage of molybdic oxide with 

 a very fair degree of approximation. In these salts the water must 

 always be determined by ignition with sodic tungstate or some similar 

 compound, since both arsenic and molybdic oxides are volatile. 



Sixteen to one Series. — When solutions of ammonic arsenate and 

 acid molybdate (7 : 3 salt) are mixed, a beautiful white crystalline pre- 

 cipitate is thrown down, which is very insohible in cold water but 

 dissolves slightly in boiling water, giving, however, a turbid solution. 

 The salt is readily soluble in ammonia water. The portion analyzed 

 was well washed on a filter with cold water and dried on woollen 

 paper. In this salt, — 



1.1322 gr. lost on ignition with WO.Na^ 0.1636 gr. NHg and HjO 



= 14.45% 

 1.3389 gr. gave 0.2481 gr. NH.Cl = 9.00 p^ (NH^P 

 1.4276 gr. " 0.1478 gr.As,0,Mg,= 7.68% AS2O5 



The analyses lead to the formula 



16 M0O3 . As A • 5 (NH^^O . H^O + 8 aq. 



which requires : — 



Calc'd. 



I6M0O3 2304 77.94 1 77.97) 



AsA 230 7.78 r^-^^ 7.68 1^^-^^ 



5(NHA0 260 8.79] ^'^^ui^ 



9 H,0 _262 5.49 j 5.45 1 

 2956 



The salt may have lost a little ammonia in drying. "When potassic 

 arsenate and acid molybdate are mixed, a similar salt is formed. A 

 solution of arsenic acid gives at once in solutions of acid ammonic or 

 potassic molybdate a beautiful white crystalline precipitate, insoluble in 

 cold water, but soluble in a large quantity of boiling water, forming 

 cloudy solutions which pass freely through a filter. These may serve 

 as starting-points for new investigations. The arsenio-molybdate above 

 described is not perceptibly altered by long boiling with nitric acid, 

 but the existence of higher compounds containing 22 or 24 molecules 

 of molybdic to one of arsenic oxide appears at least extremely probable. 



The phospho-molybdates and arsenio-molybdates now known with 

 some degree of certainty are as follows : — 



VOL. XVII. (n. S. IX.) 6 



