Page:EB1911 - Volume 10.djvu/257

Rh crystallized out it remains liquid till the temperature has fallen to its freezing point, and then consolidates simultaneously. This may be so, but analyses show that it has not always the same composition and consequently that the conditions which determine its formation are not quite simple. Felsitic rocks are sometimes silicified and have their matrix replaced by granular aggregates of cloudy quartz.

 FELSPAR, or, a name applied to a group of mineral silicates of much importance as rock-constituents. The name, taken from the Ger. Feldspath, was originally written with a “d” but in 1794 it was written “felspar” by R. Kirwan, on the assumption that it denoted a mineral of the “fels” rather than of the “field,” and this corrupted form is now in common use in England. By some of the earlier mineralogists it was written “feltspar,” from the Swedish form fältspat.

The felspar-group is divided into two subgroups according to the symmetry of the crystals. Although the crystals of all felspars present a general resemblance in habit, they are usually regarded as belonging to two systems, some felspars being monoclinic and others anorthic. Figures of the crystals are given in the articles on the different species. Two cleavages are generally well marked. In the monoclinic or monosymmetric felspars these, being parallel to the basal pinacoid and clinopinacoid, necessarily make an angle of 90°, whence the name orthoclase applied to these minerals; whilst in the anorthic or asymmetric felspars the corresponding angle is never exactly 90°, and from this obliquity of the principal cleavages they are termed plagioclase (see and ). There are consequently two series of felspars, one termed orthoclastic or orthotomous, and the other plagioclastic or clinotomous. F. E. Mallard suggested that all felspars are really asymmetric, and that orthoclase presents only a pseudo-monosymmetric habit, due to twinning. Twin-crystals are very common in all the felspars, as explained under their respective headings.

The two divisions of the felspar-group founded on differences of crystalline symmetry are subdivided according to chemical composition. All the felspars are silicates containing aluminium with some other metallic base or bases, generally potassium, sodium or calcium, rarely barium, but never magnesium or iron. The monoclinic series includes common potash-felspar or orthoclase (KAlSi3O8) and hyalophane, a rare felspar containing barium (K2BaAl4Si8O24). The anorthic series includes at one end the soda-felspar albite (NaAlSi3O8) and at the other extremity the lime-felspar anorthite (CaAl2Si2O8). It was suggested by G. Tschermak in 1864 that the other plagioclastic felspars are isomorphous mixtures in various proportion of albite (Ab) and anorthite (An). These intermediate members are the lime-soda felspars known as oligoclase, andesine, labradorite and bytownite. There are also placed in the anorthic class a potash-felspar called microcline, and a rare soda-potash-felspar known as anorthoclase.

The specific gravity of the felspars has been shown by G. Tschermak and V. Goldschmidt to vary according to their chemical composition, rising steadily from 2.57 in orthoclase to 2.75 in anorthite. All the felspars have a hardness of 6 to 6.5, being therefore rather less hard than quartz. Pure felspar is colourless, but the mineral is usually white, yellow, red or green. Certain felspars are used as ornamental stones on account of their colour (see ). Other felspars are prized for their pearly opalescence (see ), or for their play of iridescent colours (see ), or for their spangled appearance, like aventurine (see ).

Felspar is much used in the manufacture of porcelain by reason of its fusibility. In England the material employed is mostly orthoclase from Scandinavia, often known as “Swedish spar.” The high translucency of “ivory porcelain” depends on the large proportion of felspar in the body. The mineral is also an important constituent of most ceramic glazes. The melting points of felspars have been investigated by Prof. J. Joly, Prof. C. A. Doelter y Cisterich and especially by A. L. Day and E. T. Allen in the Geophysical Laboratory of the Carnegie Institute at Washington.

Among the applications of felspar is that of pure orthoclase in the manufacture of artificial teeth.

Felspar readily suffers chemical alteration, yielding (q.v.). The turbidity of orthoclase is usually due to partial kaolinization. Secondary mica is also a common result of alteration, and among other products are pinite, epidote, saussurite, chlorite, wollastonite and various zeolites.

See, , , , , ,, , , , ,.

 FELSTED, or, a village of Essex, England, between Dunmow and Braintree, and 10 m. from Chelmsford; with a station on the Great Eastern railway. Felsted is only noteworthy by reason of its important public school, dating back to its foundation as a grammar school in 1564 by Richard 1st Baron Rich, who as lord chancellor and chancellor of the court of augmentations had enriched himself with the spoil of the adjoining abbey and priory of Little Leez at the dissolution of the monasteries. It became a notable educational centre for Puritan families in the 17th century, numbering a hundred or more pupils, under Martin Holbeach (1600–1670), headmaster from 1627–1649, and his successors C. Glasscock (from 1650 to 1690), and Simon Lydiatt (1690 to 1702). John Wallis and Isaac Barrow were educated here, and also four sons of Oliver Cromwell, Robert, Oliver, Richard (the Protector), and Henry. Another era of prosperity set in under the headmastership of William Trivett (1745–1830) between 1778 and 1794; but under his successors W. J. Carless (from 1794 to 1813) and E. Squire (from 1813 to 1829) the numbers dwindled. As the result of the discovery by T. Surridge (headmaster 1835–1850), from research among the records, that a larger income was really due to the foundation, a reorganization took place by act of parliament, and in 1851, under the headmastership of Rev. A. H. Wratislaw, the school was put under a new governing body (a revised scheme coming into operation in 1876). The result under Rev. W. S. Grignon (1823–1907), the headmaster from 1856 to 1875, who may be considered almost the second founder, was the rapid development of Felsted into one of the regular public schools of the modern English type. New buildings on an elaborate scale arose, the numbers increased to more than 200, and a complete transformation took place, which was carried on under his successors D. S. Ingram (from 1875 to 1890), H. A. Dalton (to 1906), and F. Stephenson, under whom large extensions to the buildings and playing-fields were made.

See John Sargeaunt, History of Felsted School (1889); and Alumni Felstedienses, by R. J. Beevor, E. T. Roberts and others (1903).

 FELT (cognate with Ger. Filz, Du. vilt, Swed. and Dan. filt; the root is unknown; the word has given Med. Lat. filtrum, “filter”), a fabric produced by the “matting” or “felting” together of fibrous materials such as wools, hairs, furs, &c. Most textile fibres (see ) possess the quality of matting to some extent, but wools, furs and some few hairs are the only fibres which can be felted satisfactorily. It is probable that the quality of felting must be attributed to the scale structure and waviness of the wools, furs and hairs referred to. When it is desired to incorporate non-felting fibres in felt cloths, wool must be employed to “carry” them.

There are two distinct classes of felts, viz. woven or “thread-structure” felts, and “fibre” or true felts. In the manufacture of thread-structure felts, wools possessing the quality of felting in a high degree are naturally selected, carefully scoured so that the felting quality is not seriously damaged, spun into woollen yarn possessing the necessary fibre arrangement and twist, woven into cloth of such a character that subsequently satisfactory shrinking or felting may be effected, and finally scoured, milled in the stocks of machine of both, dyed and finished on the lines of an ordinary woven fabric. The lighter styles of woven felts may be composed of a single cloth only, but for the heavier styles two or more cloths are woven, one on top of the other, at one and the same time, arrangements being made to stitch the cloths together during the weaving operation.

Fibre felts are exceedingly interesting from the historical point of view. It is now generally admitted that the art of