464 



MINERALOGY. 



Oryetogno- 



V. CHARACTERS FOR THE TOUCH. 



for the 



teuch. 



~ Here we have to observe, the Hardness, the Tena- 

 Characters city, the Frangibility, the Flexibility, the Adhesion to 

 the Tongue, the Unctuosity, the Coldness, and the 

 Weight. 



XIV. The Hardness. 



The hard. The degrees are 



mss. 1. Hard. When a mineral either does not yield to 



the knife, or is very slightly affected by it, but 



affords sparks with steel, it is said to be hard. 



It is further distinguished according as it is more 



or less affected by the file. 



A. Resisting the Jile, or hard in the highest degree, 

 when it does not yield to the file, but rather 

 acts on it, as diamond, sapphire, and emery. 



B. Yielding slightly to the Jile, and very slightly to 

 the knife, or hard in a high degree, as in garnet, 

 flint, quartz and calcedony. 



C. Yielding readily to the Jile, but with difficulty to 

 the knife, or hard, as porcelain-jasper, iron-py- 

 rites, and felspar. 



2. Semihard. When a mineral gives no sparks with 

 steel, and yields more readily to the knife than 

 the preceding, it is said to be semihard, as fluor- 

 spar, and grey copper. 



3. Soft. When a mineral is easily cut by the kmte, 

 but does not yield to the nail of the finger, it is 

 said to be soft, as calcareous-spar, heavy-spar, 

 serpentine, and galena or lead-glance. 



4. Very soft. A mineral is said to be very soft, 

 when it yields easily to the knife, and also to 

 the nail of the finger, as gypsum, steatite, and 

 chalk. 



In our descriptions of minerals, it is useful to men- 

 tion their relative hardness, which is ascertained by 

 trying which will scratch the other, by drawing the 

 sharp edge or angle of one on the flat surface of the 

 other. It is, however, of consequence to know, that in 

 crystallized minerals, the solid angles and edges of the 

 * primitive forms are very sensibly harder than the an- 

 gles and edges of the derivative forms, or than the an- 

 gles and edges produced by fracture, either of crystals 

 or of massive varieties of the same species. This fact 

 has been long known to diamond cutters, who always 

 carefully distinguish between the hard and soft points 

 of this gem, that is, between the solid angles belong- 

 ing to the primitive octohedron, and those belonging 

 to any of the modifications; the latter being easily 

 worn down by cutting and rubbing them with the for- 

 mer*. Hauy, in determining the relative hardness, 

 - uses plates of calcareous-spar, glass, and quartz ; but 

 as it would be advantageous to have a more complete 

 series of minerals for ascertaining these relative de- 

 grees of hardness, I insert the following Table of Mi- 

 nerals, arranged according to their hardness, the first 

 being the hardest, the last the softest. We would recom- 

 mend to mineralogists to provide themselves with po- 

 lished pieces of these, and arrange them in a frame 

 for use. 



The following are the different degrees of hardness. 



1. Expresses the hardness of Talc. 



2. Gypsum. 



3. Calcareous-spar. 



4. Expresses the hardness of Fluor-spar. Oryciogno- 



5. Apatite. y- 



6. Felspar. "">" 



7. Quartz. 



8. Topaz. 



9. Corundum. 

 10. Diamond. 



Thus, if the hardness of a mineral is marked 7, it 

 shews that it is equal to that of quartz. If the hardness 

 is marked 7-5, it intimates that it is intermediate be- 

 tween that of quartz and the next number, 8, or topaz. 

 By using still smaller numbers, more minute degrees of 

 hardness might be expressed. 



Observations. In examining the hardness of miner- 

 als, we must be careful to attend to the following cir- 

 cumstances : 



1. Not to confound the real hardness of the mineral 

 with accidental hardness; which latter is caused by 

 the mixture of hard parts in soft minerals, and soft 

 parts in hard minerals. 



2. When minerals are composed of distinct concre- 

 tions, which are not very closely joined together, we 

 must not give the hardness of the aggregate for that of 

 the mineral, because the hardness in such cases must 

 be taken from that of the individual concretions. 



3. And we must be careful that the mineral whose 

 hardness we wish to ascertain, is not in a state of de- 

 composition. 



XIV. The Tenacity. 



By tenacity is understood the relative mobility or the The tena- 

 different degrees of cohesion of the particles of mine- city. 

 rals. There is a series from the coherent and com- 

 pletely immoveable, to the coherent and moderately 

 moveable, which latter is expressed by malleability, 

 and is the greatest degree of the mobility of the parti- 

 cles observed among solid minerals. This series con- 

 tinues through different kinds of fluid minerals, and 

 the greatest degree of the mobility of the particles is 

 found in rock-oil. The degrees of tenacity are, 



1. Briltle. A mineral is said to be brittle, when on 

 cutting it with a knife, it emits a grating noise, and 

 the particles fly away in the form of dust, and leave a 

 rough surface, which has in general less lustre than the 

 fracture. In this degree of tenacity, the particles are 

 completely immoveable. All hard, and the greater 

 number of semihard minerals are brittle. Examples, 

 quartz, heavy-spar, and grey copper. 



2. Sectile or mild. On cutting minerals possessing 

 this degree of tenacity, the particles lose their con- 

 nection in a considerable degree, but this takes place 

 without noise. The particles are coarser than in the 

 brittle variety, and do not fly off, but remain on the 

 knife. The lustre is increased on the streak. This 

 degree of tenacity occurs in most of the soft and very 

 soft minerals; and the only semihard mineral with 

 this character is native arsenic. Examples, galena or 

 lead-glance, copper-glance, graphite, and molybdena. 



3. Ductile. Minerals possessing this degree of te- 

 nacity can be cut into slices with a knife, and extend- 

 ed under the hammer. The particles are more or less 

 moveable among themselves, without losing their con- 

 nection. Examples, native gold, native silver, and na- 

 tive iron. 



* Vid. Bruckmann's Abh.ndlung von Edelst.inen, 4ter Aufl. . 88. & 29. Moh. uber Hauy's Meionite, in Von Moll'. Eferme- 

 ridtn der Berg und Huttenkunde 2ten fiandei, Iste Lief, s. 3. Aikin's Manual, p. *. 



