Orange Phosphate of Lead. 32 y 



icxnition; whereas when they are heated in the interior flame, 

 they become green by the reduction of the chromic acid 



In considering the circumstances of this analysis, it ap- 

 peared to me exu-aordinary that the chrome should be part y 

 in the state of protoxide and partly ot chromic acid ; and to 

 this point I directed my attention in examinmg the remainder 

 of the nitric solution. Instead of neutralizing it, as before, 

 I now evaporated it down, and observed the yellow colour 

 chano-e by degrees to green; ammonia was then added, aiid 

 the whole of the chrome was thrown down, no chromic acid 



being left in solution. , .u • i ^t* 



Nitric acid alone has no tendency to reduce the acid ot 

 chrome. The mineral, therefore, must have contamed some- 

 thino- which might contribute to this reduction. I heated some 

 of the crystals in a glass tube, and observed a strong empy- 

 reumatic smell indicative of vegetable matter. I then dropped 

 minute portions of sugar, green vegetable substances, oil ot 

 turpentine and bituminous coal, into solutions of the bichro- 

 mate of potash in nitric acid; by each of these, when heat 

 was applied, the chromic acid was reduced. I do not find tliis 

 effect to be produced by the gases which the action of the 

 nitric acid on vegetable matter evolves when they are passed 

 throu'rh such a solution : nitrous gas does not aflect the bi- 

 chromate of potash, neither does hydrogen in the gaseous state, 

 nor carburetted hydrogen; it is decomposed, indeed, by the 

 easeous products of the distillation of sulphuric acid and al- 

 cohol, but this reduction is not effected by the olefiant gas, 

 but by a portion of sulphurous acid which accompanies it. 



Another circumstance in the constitution of the mineral 

 may contribute to the reduction of the chromic acid. Klap- 

 roth has stated that the yellow phosphate of lead contams a 

 portion of muriatic acid; and I have found it m the specimen 

 which I have analysed. Now muriatic acid reduces the chromic 

 acid when heated with it; and in the present case, the muriate 

 of lead having been decomposed by the sulphuric acid, the 

 muriatic acid would be free to operate. 



Upon the whole, there is no reason to doubt that the chrome 

 is here united to the phosphate of lead, in the state ot chromic 

 acid, or rather of chromate of lead. "When the mineral is dis- 

 solved in nitric acid and heated, a portion of the chromic acid 

 is reduced, by one or both of the causes which I have stated: 

 if it is then neutralized with an alkali, jiart ot the chrome is 

 tlirown down as protoxide, and part remains in solution as a 

 chromate; but if the nitric solution is evaporated without 

 having been neutralized, the whole of the chrome is reduced, 

 and inav be precipitated in the state of protoxide. 



•^ ' 2 T 2 ^ \\i\\Q 



