Decomposition of [von Pyrites. 223 



These figures, however, can he trusted only when there is reason- 

 able probability, judging from the character of the matrix and 

 associations, of the absence of other impurities, even in micro- 

 scopic form, such as other metallic sulphides, quartz, iron-ochre, 

 etc., by which the density and stability might be affected. 



The natural weathering of the specimen may further be an im- 

 portant criterion of its degree of stability in masonry, etc., 

 whenever it can be ascertained that the ochreous decomposition 

 thereby shown has been effected simply by the weather upon 

 some outcrop, during periods insufficient to disintegrate the 

 whole rock, and not by the attack of subterranean solutions 

 percolating from overlying humus, or of the acid corrosive 

 agencies which may affect the material of veins or even merely 

 produce a deceptive deposit of iron-oxide upon unattacked 

 crystals of pyritg. The examination of the pyrites exposed to 

 the weather on the surface of old masonry, slating or the faces 

 of old quarries may be therefore, in many cases, far mor esatis- 

 factory for practical purposes than the study of natural outcrops 

 of the same rock, after ages of unknown experiences. Even in 

 handspecimens, or broken crystals, the long retention of color 

 and lustre, without tarnish or iridescence, for a few weeks or 

 months, on a freshly broken surface of a pyrites under trial, 

 laid out upon a window-sill, may give valuable indications of 

 stability. We thus come to the last and perhaps best means of 

 information — the process of experimental trial of the specimen, 

 side by side with a series of others of known character, by expos- 

 ure to the oxidizing effect of the fumes of bromine or of fuming* 

 nitric acid. Such tests should of course be made on several 

 samples, attention being given to the comparative rapidity of 

 tarnish, the depth of the efflorescent crust and the signs of its 

 internal penetration, the character of the etching upon any re- 

 maining nucleus, the evidences and force of any sudden explosion 

 during efflorescence, and the indications of enclosed carbon or 

 other impurity, shown by modifications of the color of the 

 effloresced vitriol. 



