Organic Acids in the Examination of Minerals. 17 



milky from the formation of an insoluble tartrate of zinc ; on 

 cooling, the solution becomes clear and a precipitate settles. 



Oxalic acid in concentrated solution attacks zinc in the 

 cold immediately ; but the action soon ceases, owing to the 

 surface of the zinc becoming coated with a quite insoluble 

 zinc oxalate ; on heating to boiling, the evolution of hydro- 

 gen is resumed, but is again arrested in a short time from the 

 same cause. On cooling, a fine precipitate forms.* 



SULPHIDES. 



12. After experiencing the solvent power of organic acids 

 described in the preceding section, we were prepared to dis- 

 cover many sulphides yielding to them, though we confess to 

 surprise at finding this action taking place even in the cold. 

 Such is the case with four out of the eighteen sulphides 

 selected for examination, viz. : stibnite, galenite, sphalerite, 

 and pyrrhotite. The tests were made as follows : the pul- 

 verized minerals were placed in test-tubes, a concentrated 

 solution of citric acid was added, and a piece of paper moist- 

 ened with plumbic acetate was suspended in each tube, which 

 was then corked. After standing twelve hours at the ordinary 

 temperature, the blackened test-papers gave evidence of the 

 decomposition. Sphalerite seems to be the most readily 

 decomposed, the sulphureted hydrogen coming off immedi- 

 ately. On heating, the disengagement of sulphureted hydro- 

 gen is very marked; with boiling citric acid, bornite and 

 bournonite, in addition to those just mentioned, yield sulphu- 

 reted hydrogen. One sample of argentite, containing min- 

 gled galenite, gave a reaction for sulphureted hydrogen ; but 



* Experiments were made to test the applicability of these methods of generating 

 hydrogen to the detection of arsenic by Marsh's apparatus. If a solution containing 

 arsenic be introduced into a flask in which hydrogen is evolved from the action of citric 

 acid on zinc, the disengagement of gas is greatly augmented and arseneted hydrogen 

 forms at once. Owing to the imperfect solubility of zinc citrate, the action soon ceases. 

 With tartaric acid tbe same objection - arises, while oxalic acid is out of the question. 

 This difficulty does not apply to magnesium, which might be employed in an apparatus 

 similar to that described by Dr. John C. Draper (American Chemist, II, 456). By using 

 distilled magnesium and re-crystallized citric acid, the absence of arsenic in the materials 

 used for toxicological researches could be placed beyond suspicion. 



July, 1877. 2 Akn. N. Y. Acad. Set , Vol. I. 



