144 Decomposition of Iron Pyrites. 



dull eroded surface, of minute bright grains, scales, pellicles, 

 and angular, sometimes branching, rods, contrasting with the 

 general surface by their brilliant lustre, yellow color, and some- 

 times a slight projection. Many consisted of barely visible 

 particles and lines, but the size of 0.005 to 0.035 mm. was 

 sometimes noticed. 



Occasionally triangular and rectangular outlines could be dis- 

 tinguished, even 0.056 mm. in length, and rarely two or three 

 faces of a flattened polished cube. Their compact bright 

 material appeared identical wdth that of the striation films, and 

 in fact, many of these forms appeared mere expansions or pro- 

 jections of these films. All the facts strengthened the idea that 

 the material of this fibrous pyrite is not uniform, as it appears 

 to the eye, but that these grains and minute lines indicate the 

 planes of successive envelopment of two materials, the one yiell- 

 ing rapidly to decomposition, the other more dense and yielding- 

 more slowly. The little pits or cavities were also closely ex- 

 amined to determine whether they ever presented symmetrical 

 outlines, which might signify the eating away of crystals of a 

 softer substance, but no such indications were recognized. 



FiftJity. The white efflorescence rarely displayed any dis- 

 tinct forms, except those of occasional needles with imperfect 

 terminations. The entire absence of any yellow or reddish tinge 

 indicated that it consisted of a basic ferric sulphate, and this 

 was confirmed by the blue reaction produced by potassium fer- 

 ridcyanide in a solution of the effloresced salt. However, when 

 fragments on a slide, immersed in a drop of potassium ferro- 

 cyanide, were examined under the microscope, traces of a blue 

 coloration were also seen, wiiich showed the presence of cop- 

 peras, ferrous sulphate, in minute amount. It would not be 

 ])ossible, without the most careful analysis, to determine the ex- 

 act character of such a natural vitriol ; since it may be consti- 

 tuted, according to the degi'ee of oxidation and hydration, of 

 indefinite mixtures of at least eleven known salts, viz., one fer- 

 rous sulphate, eight ferric sulphates, and two ferroso-ferric sul- 

 phates. 



d. The darkened sui-face of a cube from the outer surface, 

 slightly marked to the eye by minute particles of the vitriol- 

 efflorescence. This presented, under the micj'oscope, a finely 



