216 Decomposition of Iron Pyrites. 



by them. The occurrence of such differences, however excep- 

 tional among bituminous coals, seems adequate to explain the 

 cases on record in which excessively pyritic coal of this class has 

 resisted pyritic decomposition, under the same conditions known 

 in other cases to induce not only energetic pyritic weathering, 

 but spontaneous ignition of the coal. Any doubt as to this 

 point proceeds from the want of specific identification of the 

 pyrites in different coals under such circumstances as to render 

 it a question of importance." This difference in the pyrites is 

 then attributed to the distribution of marcasite as the prevailing 

 form of pyrites in the former class, with its ready yielding to 

 oxidation, and that of pyrite, with its strong resistance to de- 

 composition, in anthracite. This explanation however appears, 

 in view of my results, to be incomplete, inexact, and likely to 

 to lead to serious er'or. Marcasite, in that form, appears to be 

 a rare accessoi-y in bituminous coals. Their pyrites present 

 themselves in the crystalline form of pyrite, more or less marca- 

 sitic in internal constitution, in my view, and so passing into a 

 marcasitic paramorph after pyrite. Even in this impure pyrite 

 other sources of instability lie in its enclosure of finely divided 

 carbonaceous matter, and in the curious physical property of 

 sudden explosion under accumulated strain, during gradual vit- 

 riolization. It is therefore as unwisCj on the one hand, to trust 

 to the stability of the pyrites in a coal on the ground of its crys- 

 tallization in the isometric form of pyrite, as it is necessary, on 

 the other, to identify the true character of the mineral, by a care- 

 ful comparison of its crystalline form, exact density, and degree 

 of reaction in an oxidizing atmosphere, when subjected to ex- 

 perimental trial in one of the ways already described. 



B. Pyrites ik Buildij^^g Stone. 



On the subject of the influence of pyrites on the durability of 

 a building-stone, the most contradictory observations and state- 

 ments have been made and are now current ; the explanation 

 can be better understood in the light of the present investiga- 

 tion. 



On the one hand is the certainly established fact of the com- 

 mon distribution of varieties of pyrites, whose ready oxidation 

 produces offensive discoloration, pitting of the surface, and even 

 in some cases a disintegration of the utmost injury to a building 



