98 ON VEGETABLE AND MINERAL FORMATIONS IN CALCAREOUS SPAR. 



acid, so as to dissolve away a portion of the carbonate of lime. When the acid 

 had reduced the crystals to thin laminse, they were placed under a compound 

 microscope, with an object-glass one inch in focal length, and viewed by reflected 

 light. A number of yellow microscopic crystals were now seen scattered through 

 the transparent plate of calcareous spar. They resembled iron pyrites, and were 

 collected in great quantity in the position of two of the parallel bands. These 

 crystals were not always cubes, but most frequently short fibrous masses, inter- 

 posed in the cavities of the spar. 



" So far all was pretty plain ; but in dissolving away, by means of the hydro- 

 chloric acid, all the carbonate of lime in another portion of the specimen, instead 

 of obtaining bright yellow microscopic cubes of u-on pyrites, as I expected, the 

 residual matter consisted of small fibrous masses, with a semi-metallic lustre and 

 steel bronze colour. On analyzing them, I detected sulphur and iron, but no cop- 

 per. I was for some time perplexed by the change of appearance in the crystals 

 as seen enclosed in the matrix, and w^hen left as a residue ; and further, the 

 microscopic aspect did not agree with the result of chemical analysis. At last it 

 occurred to me that the crystals belonged to the prismatic pyrites (cock's-comb), 

 and in crushing a fragment of that mineral, and placing it under the microscope, 

 I found that its appearance exactly agreed with that of the substance under exa- 

 mination. The bronze colour of this variety of pyrites is quite superficial, and is 

 removed by the action of hydrochloric acid. It is possible that the residual crys- 

 tals may contain a little arsenic, but I had not enough of the material to pursue 

 the investigation farther." 



3. In another specimen of calcareous spar fi^om India, the extraneous matter 

 obviously consisted of minute crystals of copper pyrites. Professor Andrews found 

 that they were composed of sulphur, iron, and copper. 



