688 REPORT OF NATIONAL MUSEUM, 1897. 



F — CHARACTERS DEPENDING UPON THE ACTION OF THE SENSES. 



The touch of a mineral is described as : greasy, the feel of talc; meager, 

 dry and rough to the touch like chalk; harsh, having the feel of actin- 

 olite; smooth, having the feel like a face of a (inartz <a-ystal; adhesive 

 when it adheres to the tongue, as in the case of hydrophane. 



The taste is a character of soluble minerals only, and is described as: 

 astringent, the taste of chalcanthite; sweetish astringent, the taste 

 of kalinite; saline, the taste of halite; alkaline, the taste of natron; 

 cooliug, the taste of niter; bitter, the taste of ei)somite. 



ODOR. 



Certain minerals, under the influence of friction, moistening with the 

 breath, or by the action of heat or acids, give off odors, which are des- 

 ignated thus: bituminous, the odor of bitumen, as in elaterite; sul- 

 phurous, the odor of burning sulphur, as in pyrite when heated or under 

 friction; alliaceous, the garlic-like odor given off by arsenopyrite 

 when heated or under friction; fetid, the odor of sulphureted hydro- 

 gen given oft' under friction by some varieties of quartz and limestone; 

 argillaceous, the clayey odor given olf by kaolin upon being moistened. 



G— CHARACTERS DEPENDING UPON RESISTANCE TO CHEMICAL 



ACTION. 



CORROSION FIGURES. 



When a crystal is exposed a short time to the action of a solvent, its 

 faces are not equally attacked, but are corroded into jjit-like figures 

 whose forms obey the law of symmetry of the crystal. These figures 

 are often of assistance in determining the grade of symmetry of a crys- 

 tal, which IS not apparent by the development of its faces. Corrosion 

 figures will also indicate the presence of twins. Examples of these 

 are shown in ;i crystal of ])yrite which has been dipped in warm nitric 

 acid, and in those of calcite, boracite, barite, galena and (quartz. 



SOLUTION ri.ANKS. 



In every crystal there is a set of structure planes along which chem- 

 ical action takes place most easily. These planes have definite relations 

 with tlie symmetry of the crystal; for exauiple, the solution planes in 

 the specimen of calcite shown are parallel to the faces of a scalenohe- 

 dron. Further, in many crystals, wheu they have been subjected to 

 intense strain, planes of easy solution may arise in directions parallel 

 to that along which the strain is exerted. 



