153 



FELDSPAR 



Afterwards, on a similar occasion, he returned a 

 initial, which had l>een adjudged to him for his 

 fHH-in Providence, with the request that it might be 

 given to the poet who deserved the second . prize. 

 He tried his powers in almost every department of 

 poetry. In his earlier years, he was too much in- 

 clined to the pensive and sentimental style of Bel- 

 lamy, (q. v.) It predominates particularly hi his 

 romance Ferdinand and Constantia (1785), and, 

 throu uli his example, has for a long time prevailed 

 in Holland. His Grave is the first distinguished 

 didactic poem since the revival of Dutch poetry. 

 This poem, with a good plan, with many excellent 

 passages, and charming melody, has also too much 

 of the melancholy character. His Old Age (De 

 Ouderdom, 1802), is free from this fault, but has no 

 definite plan. Among his lyric poems, Oden en 

 Gedichten (Amst. 1798, 3 vols.) are several hymns 

 and odes distinguished for great elevation and feeling. 

 His ode on Ruyter is very celebrated. He also made 

 that naval hero the subject of an epic poem. The 

 best of his tragedies are Thirza, Johanne Gray, and 

 particularly Inez de Castro. In connexion with Bil- 

 derdyk, he gave a better form to Haren's celebrated 

 poem De Genzen, the subject of which is the founda- 

 tion of Dutch freedom. His poetical Letters to 

 Sophia on Kant's Philosophy (Brieven aan Sophie over 

 de Kantiaansche JVtjsbegeerte, Amst. 1805) are a 

 feeble effort of his old age. Among his prose 

 works, his Letters on different Subjects of Literature 

 (6 vols. 1784) are distinguished, and contributed 

 much to the dissemination of good taste, by their 

 finished style and excellent criticisms. 



FELDSPAR ; a name in mineralogy, under which 

 has been comprehended a great variety of substances, 

 hitherto believed to form a single species, but which 

 the researches of modern mineralogists prove to 

 constitute several distinct species. The inquiries 

 upon which the proposed distinctions depend, how- 

 ever, being among the nicest in the science, cannot, 

 consistently with the general plan of this work, be 

 noticed here. We shall rather confine our remarks 

 to tliat portion of the contents of the old species of 

 feldspar, in which, from its wide distribution and 

 known applications, mankind are more generally in- 

 terested. Its crystals and crystalline masses yield 

 to cleavage parallel to the planes of a doubly oblique 

 prism, which presents, by the reflecting goniometer, 

 in one direction, four angles of 90 ; in another, four, 

 alternately of 59 25' and 120 35'; in another, four, 

 alternately of 67 15' and 1 12 45' the two cleav- 

 ages, which are perpendicular to each other, being 

 obtained with the greatest facility, while the thira 

 is effected with much difficulty. One of the perpen- 

 dicular cleavages is effected with greater ease than 

 the other, indications of which are always apparent 

 in delicate, parallel lines upon the faces produced 

 by the less distinct cleavage. The general figure of 

 the numerous crystals of feldspar is an oblique prism, 

 with unequally produced planes, whose number 

 varies from four to ten. These prisms ar > termin- 

 ated by summits, composed, ordinarily, of two large, 

 culminating faces, and several smaller faces, which 

 seem to obey no constant law of arrangement. 

 Hence it results, that the forms of feldspar are 

 among the most difficult to understand and describe 

 of those found in any species in mineralogy. The 

 following may be instanced as the simplest of those 

 ordinarily met with, viz., an oblique prism with four 

 faces (Felspath unitaire, H.); a prism with ten faces, 

 six broad and four narrow, terminated at each ex- 

 tremity by two broad culminating faces (F. tjuadri- 

 decimal, H.); an oblique rhombic prism (oblique 

 from the obtuse edge,) hnving its acute lateral edges 

 truncated and terminated by a single plane at each 



extremity (F. prismatique, II.) ; the same as the lact, 

 but terminated at each extremity by summits of five 

 faces, disposed without symmetry (Fl. sea-decimal, H.) 

 The lustre of feldspar is vitreous, sometimes inclining 

 to pearly, upon the perfect faces of cleavage ; pre- 

 vailing colour white, inclining to gray or red; some- 

 times grey, flesh red, and rarely verdigris green ; 

 translucent, and sometimes transparent, and occa- 

 sionally offers a bluish opalescence in certain direc- 

 tions ; hardness below quartz, but not scratched by 

 the knife ; specific gravity from 2.53 to 2.60. It is 

 not common to find feldspar in distinct crystals. Its 

 more usual mode of occurrence is in broad, foliated 

 masses, variously disseminated among other minerals. 

 Rarely it occurs in granular concretions ; and occa- 

 sionally, it is quite compact. Before the blowpipe 

 upon charcoal, it becomes glassy, semitransparent, 

 and white, but melts only with difficulty, on its edges, 

 into a semi-transparent vesicular glass. A crystal- 

 lized specimen, analyzed by Vauquelin, gave silica, 

 64; alumine, 20; potash, 14; and lime, 2. Feld- 

 spar is the most generally diffused, both as to its 

 local and geological situation, of all minerals, with 

 perhaps the exception of quartz. It is an essential 

 constituent of granite and gneiss, and frequently 

 occurs in micaceous and argillaceous slate. 1 1 is con- 

 tained abundantly in almost all porphyries, in which 

 it sometimes exists in large imbedded crystals. It 

 abounds in primitive and secondary greenstone, the 

 traps and trachytes, forms a large part of lavas, and 

 has even been recognised as an ingredient in many 

 meteoric stones. It is occasionally, though rarely, 

 found in veins, or beds, in primitive limestone ; and 

 though, when occurring along with quartz and mica, 

 in the primitive rocks, it is most generally dissemin- 

 ated, yet it frequently forms concretions separated 

 from those ingredients, assuming the shape of more 

 or less extended, irregular beds. If these be decom- 

 posed, by the action of the air, beds of porcelain 

 earth are formed, the most remarkable of which are 

 those in gneiss, at Aue, near Schneeberg, in Saxony, 

 and at Hafnerzell, in the district of Passau. Similar 

 deposits occur near Limoges, in France, and in Corn- 

 wall, in England. Localities of it are known in 

 America and in China, where it is called kaolin. 



Several varieties of feldspar are used in the arts 

 and manufactures. 1. The transparent, opalescent 

 variety, from Ceylon and St Gothard (commonly 

 called adularia), is much esteemed in jewelry. When 

 cut en cabochon, it reflects, from its interior, a 

 pearly, white light, which floats from one part of its 

 surface to another, according as we vary its position ; 

 from which circumstance it is called the moon-stone, 

 or fisK's-eyestone. It is often mounted in the centre 

 of a circle of diamonds, whose sparkling reflections 

 contrast in a beautiful manner with the silvery light 

 hovering over the moon-stone. 2. The verdigris-green 

 variety, called the amazon-stone, which comes from 

 near Ekaterinbourg, in Russia, and which has also 

 been found in small quantity at Beverly, in Massa- 

 chusetts, is likewise much esteemed by the lapidary. 

 3. A third variety of this species, employed in 

 jewelry, is the avanturine feldspar, which comes 

 from the island of Cedlovatoi, near Archangel, and 

 which is of a honey-yellow colour, end every where 

 penetrated by little golden spangles. 4>. The pure 

 varieties of feldspar are used in the composition of the 

 paste of porcelain ; also for the enamel with which 

 it is covered ; and the decomposed variety, or por- 

 cplain earth itself, is the most important material in 

 that department of manufactures. See Porcelain. 

 The substances formerly known under the names of 

 siliceous feldspar and aliiite, and which have generally 

 been embraced under the present species, were sepa- 

 ated, by Mr Brooke, and erected into a distinct 



