20 Organic Acids in the Examination of Minerals. 



acid than in water. Apatite is feebly attacked on boiling. 

 Vivianite is readily soluble. Pyromorphite and anglesite are 

 partly decomposed even in the cold, as shown by the behavior 

 of the nitrates with hydrosulphuric acid. 



Fluorite, cryolite, and samarskite are not attacked, as 

 might be anticipated. 



SILICATES. 



16. Those silicates which are decomposed by hydrochloric 

 acid, either with or without the formation of a jelly, are like- 

 wise attacked more or less strongly by a hot solution of citric 

 acid. The minerals were examined as follows :— their behavior 

 with strong hydrochloric acid was first ascertained, and they 

 were then heated with a concentrated solution of citric acid; 

 the solution becomes viscid by concentration, and the gelatin- 

 ous silica is best seen by diluting with water and agitatiug. 

 Very careful pulverization of the mineral is in most cases 

 indispensable. The solubility of the mineral was also tested 

 by filtering from the silica and undissolved material, and 

 examining the solution with an appropriate reagent. Owing 

 to the power possessed by citric acid (in common with other 

 organic bodies) of preventing precipitation of salts which are 

 otherwise insoluble, care was had to select those reagents 

 which were least affected ; in this we were aided by a table 

 contained in Dr. Hermann Grothe's paper entitled, " Ueber 

 das Verhalten der Metalloxydauflosungen gegen Alkalien bei 

 Gegenwartnicht-fliichtiger organischer Substanzen und ueber 

 den Nachweis der Metalloxyde in solchen Losungen." * 



So far as possible, color reactions were employed. 



The results are summarized below : — 1, signifying minerals 

 which decompose readily ; 2, those which are attacked with 

 difficulty; 3, those which resist the acid. .Minerals which 

 yield gelatinous silica are marked G, and those which give 

 slimy or pulverulent silica, S. 



* Journal fur prakt. Chemie, Vol. XCH, p. 175. 



