MINERALOGY. 



447 



O/jetogno- we look on it* surface, but when held between the 

 eye aud the light is wine-yellow. 



V. The Iridescence. 



The irid- When mineral exhibits the colours of the prism or 



oct. the ra'iibow, arranged in parallel, and sometimes vari- 



ri li)t-r, it it said to be iridescent. It is 



to IK- ohatrved uy 



A. Looking '/ the mineral only, as in precious opal, 



a'tu'tr a, \c. 



B Bih by looking on the mineral and through it, as 

 in calcareous spar, crossed by thin veins, some arra- 

 gomtes, rainbow cakcdony, and some amethysts. 



VJ. Tarnuhtd Colour*. 



Tarniihed A mineral is said to be tarnished, when it shews on 

 fears. its external surface, or on that ot the distinct concre- 

 tions, fixi-.l colours different from those in its interior 

 or fresh fracture. 



There are simple or variegated tarnished colours. 

 a. Simple. 



*. d'-cy, white cobalt. 

 ft. Black, native arsenic. 



<wi, magnetic pyrites. 

 >. Rftldiih, native bismuth. 

 6. l't;> legated. 



The variegated or party-coloured, are distinguished 

 according to the intensity of their basis. Of these the 

 following are enumerated in the tabular view. 



. I'acwinf, or Peacock-tail larniih. This is 

 an a>cmblage of yellow, green, blue, red, 

 and brown colours, on a yellow ground. The 

 colours are nearly equal in proportion, and 

 are never precisely distinct, but always pass 

 more or less into one another. Example, cop- 

 per-pyrites. 



ft. Iridftcfnt, or Rainboir. In this variety the 

 colour* are red, blue, green, and yellow, on 

 a grey -ground. It if more beautiful and brighter 

 than the preceding. The radiated grey anti- 

 mony of Feltortanya in Hungary, and the spe- 

 cu'ar iron-ore, or iron-glance of Elba, are often 

 beautifully iridescent. 



y. Culu + b'ne, or pigeon-neck larnuh. The co- 

 lour* are the same a* in the preceding, with 

 this difference, that tin- tints of colour are paler, 

 and the red predominates. Example*, Native 

 bismuth of S< hneeberg. 



J. Trmf)ertd- 're ' lamith. It nmMSti of very pale 

 blue, red, green, and very little yellow, on a 

 grey ground. Example, grey cobalt. 



VII. The Permanent Alteration*. 



Th p*rm- These must not be confounded with the tarnished 

 nt alura- colours. The tarnish occurs only on the surface ; the 

 * permanent alteration, on the contrary, proceeds by 



degreei through the whole HUM of the mineral. This 

 change takes place more or le* rapidly in different 

 minerals. The eolours either become paler, when 

 they are said ' they lieconie darker, and pass 



in'o other varieties. Thu chryopra.e, rose quartz, 

 ami red cobalt-ochre become paler ; whereas sky-blue 

 fluor-.par becomes green, pearl.grey corneous silver 

 somrtimes change* to brown, and lastly into black, and 

 earthy blue iron changes from white, through different 

 varieties ot blue, to indigo-blue. 



External Shape. 



PARTICULAR GENERIC EXTERNAL 

 CHARACTERS. 



I PARTICULAR OEVER1C EXTERNAL CHARACTERS OF 



SOLID MIXhHAL-. 



Character* for the Sight. 



i 



1. The External Aspect. 



The External Aspect of a mineral is that outline or 

 contour which it has received from nature. Thus, if 

 we have a piece of lead-glance, as it has been found 

 loose, or imbedded in another mineral, we name the 

 surface which it has received from nature, its Aspect. 

 All those characters whic : we can discover by the eye, 

 on this outline, are denominated the External Aspect 

 of the mineral. They are of three kinds : 1. The Ex- 

 ternal Shape: 2. The External Surface, and, 3. The 

 External Lustr*. 



1. The External Shape. 



Is divided into four classes, 



1. Common ) 



2. Particular f 

 S. Regular f ' 

 4. Extraneous J 



All of these classes have their subordinate differen- 

 ces, which we shall now describe ; and, 



I. Common External Siape. 



Common External Shapes are those in which there Common 

 are neither a determinate number of planes meeting external 

 under determinate angles, nor any resemblance to *'>(* 

 known natural or artificial bodies. As they occur more 

 frequently than the other shapes, they are named Com- 

 mon External Shapes. 



Six different kinds are enumerated by Werner, which 

 re distinguished according to their relative length, 

 breadth, and thickness, their relative magnitude, and 

 their connections with other minerals. The kinds are 

 inamif, dint initial *-(t, in angular ptecet, in grain*, in 

 p'atet, and IH tntu.branct. 



A. Muting, is that common external shape which is 

 trom the si>e ot a h izel-nut to the greatest magni- 

 tude, and whose dimensions in length, breadth, and 

 thickness, are nearly alike. It occurs imbedded in 

 other minerals, and it is intermixed with them at 

 their line of junction. Examples, Galena or lead- 

 glance and copper pyrites. 



Many of the varieties of this form are crystallized, al. 

 though they do not appear so to the eye. This cu- 

 rious fact has been lately well elucidated by Mr. 

 1 ) inn II. He remarks, that if a lump or massive 

 piece of alum, or borax, or of nitre, be immersed in 

 a vessel of water, and left at rest for three or four 

 weeks, the solution will be found to have gone un- 

 equally on ; the uppermost portion will be found 

 mo-t wasted, and the undermost least ; so that the 

 undiasolved part of these salts will have assumed a 

 conical lorm. The lower part of these bodies, after 

 this treatment, will be found embossed over with nu- 

 merous crystalline forms. These in alum are octa- 

 hedrons, or figures formed by different sections, of 

 the aluminous octahedron. In borax they are frag- 



