853 



MINERALOGY. 



MOCHA STONE. 



854 



tion ; and those in which they are unequal (the trimetric, mouooliuate 

 and triclinate systems), have two axes of double refraction. 



Both rays in the latter are rays of extraordinary refraction. In 

 nitre the two axes are inclined about 5" to each other ; in arragonite 

 18" 18' ; in topaz 65. The positions of the axes thus vary widely 

 in different minerals. 



The extraordinary ray exhibits a peculiar property of light, termed 

 Polarisation. Viewed by means of another doubly-refracting crystal, 

 or crystalline plate (called from this use of it an analysing plate), the 

 ray of light becomes alternately visible and invisible as the latter 

 plate is revolved. If the polarised light be made to pass through a 

 crystal possessed of double refraction, and then be viewed in the 

 manner stated, rings of prismatic colours are developed, and on 

 revolving the analysing plate the coloured rings and intervening dark 

 ring successively change places. 



Several minerals give out light either by friction or when gently 

 heated. This property of emitting light is called Phosphorescence. 



Two pieces of white sugar struck against one another give a feeble 

 light, which may be seen in a dark place. The same effect is obtained 

 on striking together fragments of quartz, and even the passing of a 

 feather rapidly over some specimens of zinc-blende is sufficient to 

 elicit light. 



Flour-spar is the most convenient mineral for showing Phosphor- 

 escence by Heat. On powdering it, and throwing it on a shovel heated 

 nearly to redness, the whole takes on a bright glow. In some varieties 

 the light is emerald-green ; in others purple, rose, or orange. A 

 massive fluor from Huntington, Connecticut, shows beautifully the 

 emerald-green phosphorescence. 



Some kinds of white marble, treated in the same way, give out a 

 bright yellow light. 



After being heated for a while the mineral loses its phosphor- 

 escence ; but a few electric shocks will in many cases to some degree 

 restore it again. 



Many minerals become electrified on being rubbed, so that they will 

 attract cotton and other light substances; and when electrified some 

 exhibit positive and others negative electricity when brought near a 

 delicately suspended magnetic needle. The diamond, whether polished 

 or not, always exhibits positive electricity, while other gems become 

 negatively electric in the rough state, and positive only in the polished 

 state. Friction with a feather is sufficient to excite electricity in some 

 varieties of blende. Some minerals thus electrified retain the power 

 of electric attraction for many hours, as topaz, while others lose it in 

 a few minutes. 



Many minerals become electric when heated, and such species are 

 said to be Pyro-Electric. 



If a prism of tourmaline, after being heated, be placed on a delicate 

 frame, which turns on a pivot like a magnetic needle, on bringing a 

 magnet near it, one extremity will be attracted, the other repelled, 

 thus indicating the polarity alluded to. Several other minerals exhibit 

 electrical phenomena, especially boracite and topaz, which, like tour- 

 maline, are hemihedral in their modifications. 



Magnetism is exhibited more especially in the ores of iron. The 

 loadstone, as the magnetic oxide of iron is called, is common where 

 ths ores of iron are found. When mounted like a horse-shoe magnet, 

 a good loadstone will lift a weight of many pounds. This is the only 

 mineral that has decided magnetic attraction ; but several ores con- 

 taining iron are attracted by the magnet, or, when brought near a 

 magnetic needle, will cause it to vibrate ; and moreover, the metals 

 nickel, cobalt, manganese, palladium, platinum, and osmium, have been 

 found to be slightly magnetic. 



Minerals vary in their specific gravity. This must be ascertained 

 as for any other substance. [SPECIFIC GRAVITY, in ABTS AND 

 Sc. Div.] 



The Hardness of minerals differs much, and is the point first attended 

 to by the mineralogist. In order to ascertain the hardness of a mineral 

 it is only necessary to draw a file across the specimen, or to make trials 

 of scratching one with another. As standards of comparison, the 

 following minerals have been selected, increasing gradually in hardness 

 from talc, which is very soft and easily cut with a knife, to the 

 diamond, which nothing will cut ; this table is called the Scale of 

 Hardness : 



1. Talc, common foliated variety. 



2. Rock-Salt. 



3. Calc-Spar, transparent variety. 



4. Fluor-Spar, crystallised variety. 



5. Apatite, transparent crystal. 



6. Felspar, cleavable variety. 



7. Quartz, transparent variety. 



8. Topaz, transparent crystal. 



9. Sapphire, cleavable variety. 

 10. Diamond. 



If on drawing a file across a mineral it is impressed as easily, as 

 fluorspar, the hardness is said to be 4 ; if as easily as felspar, the 

 ,. sa is said to be 6; if more easily than felspar, but with more 

 difficulty than apatite, its hardness is described as 5J or 5 '5. 



The file should be run across the mineral three or four times, and 

 care ihould be taken to make the trial on angles equally blunt, and 

 on parts of the specimen not altered by exposure. Trials should also 



be made by scratching the specimen under examination with the 

 minerals in the above scale, as sometimes, owing to a loose aggregation 

 of particles, the file wears down the specimen rapidly, although the 

 particles are very hard. 



Minerals differ in their state of aggregation. Solid minerals may be 



Brittle t'when parts of the mineral separate in powder on attempting 

 to cut it. 



Sectile : when thin pieces may be cut off with a knife, but the minera 

 pulverises under a hammer. 



Malleable : when slices may be cut off, and these slices will flatten 

 out under the hammer, as native gold and silver. 



Flexible : when the mineral will bend, and remain bent after the 

 bending force is removed, as talc. 



Elastic : when after being bent it will spring back to its original 

 position, as mica. 



A Liquid is said to be Viscous when on pouring it the drops lengthen 

 and appear ropy, as petroleum. 



When a mineral is broken its cut surface presents different aspects. 

 The following are the several kinds of fracture in minerals : 



Conchoidal : when the mineral breaks with a curved or concave and 

 convex surface of fracture. Flint is a good example. 



Even : when the surface of fracture is nearly or quite flat. 



Uneven : when the surface of fracture is rough with numerous small 

 elevations and depressions. 



Hackly: when the elevations are sharp or jagged, aa in broken iron. 



Soluble minerals may have taste : the kinds are 



Astringent : the taste of vitriol. 



Sweetish-astringent : the taste of alum. 



Saline : taste of common salt. 



Alkaline : taste of soda. 



Cooling : taste of saltpetre. 



Bitter : taste of Epsom salts. 



Sour : taste of sulphuric acid. 



Excepting a few gases and soluble minerals, minerals in the dry 

 unchanged state do not give off odour. By friction, moistening with 

 the breath, the action of acids, and the blow-pipe, odours are some- 

 times obtained, which are thus designated : 



Alliaceous : the odour of garlic. It is the odour of burning arsenic, 

 and is obtained by friction and more distinctly by means of the blow- 

 pipe from several arsenical ores. 



Horse-Radish odour : the odour of decaying horse-radish. It is the 

 odour of burning selenium, and is strongly perceived when ores of 

 this metal are heated before the blow-pipe. 



Sulphureous : odour of burning sulphur. Friction will elicit this 

 odour from pyrites, and heat from many sulphurets. 



Fetid : the odour of rotten eggs or sulphuretted hydrogen. It is 

 elicited by friction from some varieties of quartz and limestone. 



Argillaceous, the odour of moistened clay. It is given off by 

 serpentine and some allied minerals when breathed upon. Others, aa 

 pyrargillite, afford it when heated. 



Without submitting the mineral to a regular analysis, advantage is 

 often taken of the effects of heat by meaus of the blow-pipe, with or 

 without the aid of certain fluxes, as soda, phosphoric salt, &c. ; and the 

 mineral is stated to be either fusible alone, or with the assistance of 

 the different fluxes, and the nature of the resulting compound is 

 described ; sometimes it is a colourless glass, at other times coloured, 

 transparent, or opaque, &c. [BLOW-PIPE, in ARTS AND Sc. Div.J 



(Dana, . Manual of Mineralogy ; Dana, A System of Mineralogy ; 

 Ansted, Elementary Course of Geology, Mineralogy, &c. ; Phillips, 

 Introduction to Mineralogy ; Phillips, Elements of Mineralogy ; Jack- 

 son, Minerals and their Uses ; Sowerby, Popular Mineralogy.) 



MINIM. [LEUCISCUS.] 



MINIUM. [LEAD.] 



MINK, a name for the Vison-Weasel, Muatela (Pulorius) Viaon. 



[MUSTELiD.fi.] 



MINNOW. [LEUCISCCS.] 



MINT. [MENTHA.] 



MINX, a name for the Vison-Weasel. [MuSTELlD^;.] 



MINX-OTTER, Pennant's name for the Visou-Weasel. [MusiEHD^:.] 



MINYAS. [ACTINIAD.E.] 



MINYAS, a genus of Echinodermota. 



MIRE-CROW, a name for the Laughing GulL [LAHID.E.] 



MIRE-DRUM, a name for the Bittern (Bolaurus stettaris). [BiTTEUN.] 



MIROUNGA, Dr. Gray's name for a genus of Seals. [PuociDiE.] 



MISCOPHUS. [LARID.E.] 



MI'SILUS. [FORAMINIFEEA.] 



MISPICKEL. [IRON.] 



MISSEL-THRUSH. [MERULnxB; THRUSHES.] 

 MISSELTOE, or MISTLETOE. [Viscum.] 



MISTONUSK, one of the Cree Indian names for the American 

 Badger (Melee Labradoria, Sab.). [BADUER.] 

 MITE. [ACARIDJE.] 

 MITHRAX. [MAIID^.] 

 MITRA. [VOLUTION;.] 



MITU, a name for some of the Curassow Birds. 

 MNEMIA. [AcALEraa:.] 



MOA. [DlNORNIS.] 



MOCHA-STONE. [AQATE.] 



