l)econ\po>iitwn of Iron Pyrites. 161 



Tlje SMine i-esults ;ire dispUived in this series as in the j)i'eced- 

 ing. uith no Hpi)ai-ent advantage from the change of reagent. 



B. Microscopical Examixatiox of Etched Nuclei. 



The evidences already obtained have indicated the intermix- 

 ture, in most specimens of iron-pyrites, of two species, the one 

 strongly resistant, the other readily submitting to any process 

 of oxidation by either natural or artificial agency. The ques- 

 tion was thereby suggested whether the surface of a pyrite- 

 crystal undergoing such process of corrosion might not exhibit 

 minute cavities corresponding to the crystalline form of the par- 

 ticles of the more readily attacked mineral, marcasite, supposed 

 to be enclosed. The surfaces of the nuclei or undecomposed 

 I'emnants of pyrite, referred to in the last columns of the two 

 preceding tables, were therefore carefully examined under low 

 magnifying povvers, up to 200 diameters, on the stage of a mi- 

 croscope. Occasionally some rhombic forms, more or less indis- 

 tinct, were noticed in the cavities on some of these pitted surfaces, 

 but none wliich could be with certainty assigned to the removal 

 of marcasite-crystals. The i-esults of the microscopical exami- 

 nation of these artificially etched surfaces thei-efore correspond 

 essentially to those obtained from the weathered and decayed 

 specimens of jiyrite, already reported. In most cases, it is 

 probable, the intermixture is far too intimate and minute for 

 identification in this way— it may be, even molecular ; the nor- 

 mal ciTstalline form may also have been rarely developed in 

 grains enclosed under such conditions. But the examination, 

 though unsuccessful in its direct object, brought out facts of 

 much interest concerning the internal structure of these speci- 

 mens, of which the principal may be briefly summed up in the 

 following Table; 



