1894.1 



231 



[Brown. 



cate a third or fourth was tried. The tendency of the marcasite to cake, 

 noted in the previous series became still more marked here, and is doubt- 

 less the cause of one of the fuur-hour oxidations (marked by parenthesis) 

 being notably lower than the three-hour. Such a result is obviously 

 incorrect. On the other hand, the result in the three-hour column which 

 is placed in parenthesis is the highest obtained. This experiment was 

 made at the same time as the one showing 1.93 per cent., but the room 

 was very warm (25°), which may in part account for this high result. It 

 will be noticed that the oxidation of the pyrite seems to stop at the three- 

 hour trial, those following showing no appreciable increase. This is well 

 seen in the graphic representation of these oxidations (Pis. xvii and xviii). 



Table Showing the Relative Oxidation of Sulphur in Pyrite and Marcasite 

 by a 1 Per Gent. Solution of KMnO^ at 220. 



Action of 3 Per Cent. Solution op Potassium Permanganate at 

 Ordinary Temperature. 



The conditions of this series of experiments were the same as those 

 of the last. The tendency of the results to fluctuate instead of show- 

 ing a gradual progression is now very marked. One of the one-hour 

 pyrite oxidations shows more sulphur oxidized than is shown by any 

 other individual result of the series. No explanation can be offered for 

 such a discrepancy as this. On the other hand, the high result shown in 

 the three-hour oxidation is quite easily explained by the marcasite hav- 

 ing been little, if any, caked in this experiment. The two low results of 

 pyrite three hour and marcasite four-hour oxidations are readily explica- 

 ble on the ground of caking of the material. As the barium sulphate 

 was often determined several days after the oxidation was completed, it 

 is obvious that no reliable notes could be made concerning the caking or 

 non caking of the mineral in the permanganate. "With this strength of 

 solution it is evident, too, that the main action of the permanganate is 

 complete at the end of one hour, notably in the case of the marcasite, and 

 it is only when very vigorous agitation exposes fresh surfaces of the min- 

 eral to the action of the KMnO^ that any further action can take place. 

 We therefore see that marcasite in one hour gives up as much sulphur as 

 in live hours, and this is very graphically shown on Pl. xvii. 



PROC. AMER. PHILOS. SOC. XXXIII. 145. 2 D. PRINTED JUNE 18, 1894. 



