Organic Acids in the Examination of Minerals. 25 



mineral ; a violent action sets in immediately. In studying 

 the rationale of the above-named decompositions, we discov- 

 ered that many sulphides are attacked by an aqueous solution 

 of potassium nitrate (and nitrite) without the addition of acid; 

 this unexpected result explains satisfactorily the violence of 

 the decomposition of the sulphides by the mixture of reagents, 

 many forces being simultaneously brought into play; the 

 organic acid, while able to decompose the potassium nitrate 

 aloue, acts at the same time as a solvent of the products 

 resulting from the decomposition of the sulphide by the 

 nitrate. 



This explains also the fact that the comparatively weak 

 acids — malic, acetic, and formic— which do not of themselves 

 decompose potassium nitrate, are able to dissolve the sulphides 

 in the presence of the latter reagents. We are now investi- 

 gating the behavior of certain minerals with these and other 

 saline solutions, and hope to present the results in a future 

 paper. 



We have applied this method of attack to several other 

 classes of minerals, but without much advantage. The oxides 

 of iron, magnetite and hematite, as well as franklinite and 

 chromite, resist the combined action of the mixed reagents. 

 The decomposition of silicates is not notably facilitated. 

 Limonite is feebly attacked. Uraninite, however, dissolves 

 completely and rapidly in this mixture of reagents. 



The remarkable solvent power of a mixture of nitrate of 

 potassium and citric acid is further demonstrated by the fact 

 that metallic copper, silver, lead, tin, bismuth, and antimony, 

 as well as magnesium, iron, and zinc, dissolve therein with 

 more or less rapidity. 



Aluminium resists this mixture of reagents : — the alchem- 

 ists' dream of an alcahest, or universal solvent, is therefore 

 only partly realized. 



DECOMPOSITION OF SILICATES BY ORGANIC ACIDS AND 

 AMMONIUM FLUORIDE. 



21. A hot concentrated solution of citric acid decomposes 

 ammonium fluoride, setting hydrofluoric acid free ; and if 



