MINERALOGY. 



469 



Orrctogno- excess of alkali by nitrous acid, then evaporating the 

 *! whole to dryness, and, lastly, hastily melting it in a 



S " Y ~"' platina crucible. For an active, and at the same time 

 non-alkaline flux, boracic acid may be used, or neutral 

 borate of soda ; and where a slight excess of alkali is 

 required, or at least does no harm, common borax by 

 iuelf, or mixed with a little cream of tartar, when a 

 strong reducing flux is required, may be had recourse 

 to. For coloured glasses, the proper support is leat- 

 platina, but for reductions, charcoal. In the latter 

 case, the ore, previously roasted, if it contain either 

 sulphur or arsenic, is to be pulverised, and accurately 

 mixed with the flux ; a drop of water being then added 

 to make it cohere, it is to be formed into a ball, and 

 deposited in a shallow hole in the charcoal, being also 

 covered with a piece of charcoal, if a high degree of 

 heat is required. In the easily reducible metals, a bit 

 of the ore being placed in the charcoal, and covered 

 with glass of borax, will, in the space of a few seconds, 

 be melted by the blowpipe, and converted into a me- 

 tallic globule, imbedded in a vitreous scoria. In all 

 cue* where a metallic globule is obtained, it should be 

 separated from the adhering scoria, and examined as to 

 its malleability, and other external characters ; being 

 then placed a' second time on the charcoal, but without 

 (lux, it is to be brought to the state of a gentle ebulli- 

 tion, during which, the surface being oxygenated, will 

 exhale a heavy vapour, that condenses on the blowpipe, 

 or falls down on the charcoal in the form of a powder, 

 or of specular crystals, from the colour and other cha- 

 racters of which, the nature of the metal may probably 

 be ascertained. If any suspicion is entertained of a 

 portion of silver or of gold being mixed with the oxyd- 

 able metal, the button must be placed on an earthen 

 support, and there brought to a full melting heat : by 

 Jegrfcs. the oxydable metal will become scorified, and 

 will entirely sink into the support, leaving on the sur- 

 face a bright bead of fine metal, if any such was con- 

 tained in the alloy ; but the proportion of this last be- 

 ing generally very small, and the entire -mass of the 

 alloy often not exceeding a large shot, it is not unfre- 

 quently necessary to have recourse to the magnifying- 

 glass to be fully convinced of the presence or absence 

 of the fine metal . 



PHYSICAL CHARACTERS OF MINERALS. 



fhjriicii 1'hytical characters, are those derived from phy- 

 1 sical phenomena originating from the mutual action of 



yj i ' minerals and other bodies. They are highly curious in 



a general view, but are seldom useful in the discrimi- 

 nation of mineral!), as they occur in but few species, 

 and in these rare cases the same physical properties are 

 met with in very different species. The principal phy- 

 sical characters which occur among minerals, are Elec- 

 tricity, Magnetism, and Phosphorescence. 



1. Electricity. 



Electricity. It is well known that there are two kinds of electri- 

 city ; the one named positive or vitreous, and the other 

 negative or resinous. 



Electricity can be excited in minerals in three diffe- 

 rent ways, by friction, by heating, or by communica- 

 tion with an electrified body. The greater number of 

 mineral* which are capable of becoming electrical, ac- 

 quire this property by friction. Earthy, saline, and 

 metallic minerals, in this way become positively elec- 

 trified. Some minerals by this process become very 



easily and powerfully electric, while others become Oryctogno- 

 electric with difficulty, and exhibit but faint traces of * y , 

 it. A few minerals become electric by heating, and S-P "V^- / 

 these belong to the number that also exhibit electrical 

 properties by friction. It has been ascertained, that 

 these minerals have at least two points, of which the 

 one is the seat of positive, and the other that of nega- 

 tive electricity. To these points, which are always 

 placed in two opposite parts of the mineral, Hauy gives 

 the name of electric poles. For a full account of the 

 pyro-electricity of minerals, see the article ELECTRI- 

 CITY, vol. viii. p. 4-56. 



The third mode of exciting electricity in minerals, 

 or that by communication, occurs only in minerals 

 which are in a pure metallic state. 



' lagnetism. 



Very few minerals are magnetic; it is a character Magnetism, 

 which occurs principally in ores of iron, or in such 

 minerals as contain a portion of metallic iron, or iron in 

 the state of black oxide. A good many minerals, after 

 exposure to the blow-pipe, become magnetic. 



3. Phosphorescence. 



Some minerals, when rubbed or heated emit in the Phosphor- 

 dark a more or less shining light, are said to be phos- cscence. 

 phorescent. Thus, yellow blende, when scratched 

 with a hard body, emits a strong light. When two 

 pieces of quartz are forcibly struck against each other, 

 both become luminous; and fliior spar, and other mi- 

 nerals, when heated, become phosphorescent. For a 

 full account of phosphorescent minerals, by Dr. Brew- 

 ster, see the Edinburgh Philosophical Journal, vol. i. 

 p. 383. 



Geognottical and Geographical Characters. 



These characters are derived from the geognostical Geognosti- 

 relations and geographical distribution of the species. cal nd 

 In another article we shall shew, that simple minerals 8 e g r ph'- 

 are not irregularly distributed throughout the crust of ^ ch r c - 

 the earth. On the contrary, that particular species 

 very often occur together, and in the same formation ; 

 that some species are met with in nearly all the forma- 

 tions of rocks, others in only a few members of the 

 series, while some are confined to a particular rock; 

 and that certain beds and formations are characterised 

 by the simple minerals they contain. In their geogra- 

 phical distribution, numerous interesting relations will 

 be pointed out, of the grouping of particular species in 

 limited tracts of country ; of the wider range of others 

 through whole regions ; and of the distribution of 

 species according to distance from the equator, and 

 particular meridians. 



Moht Crystallography. 



The preceding details make us acquainted with those Moh* 

 characters which are used in describing minerals ; but crytuii 

 iu the following arrangement considerable use is made K"P h 3 r 

 of the primitive forms, as ascertained by Professor 

 Mohs. According to his view, every simple form, 

 from which other simple forms are derived, is named a 

 fundamental form ; and the class of figures derived 

 from that fundamental form, a system of crystallizations. 

 The fundamental forms are four in number, and, con- 

 sequently, also the systems of crystallization. Thesefun- 

 damental forms are, the rhomboid, pyramid nilh a square 

 base, oblique four-sided prism, and hexahedron or cube; 



Thm OtMerration* on tht action of the Blowpipe are taken ftom Mr. Aikin'i Manual, p. 35. 



