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 CLAUSIUS, RUDOLF JULIUS EMMANUEL (1822–1888), German physicist, was born on the 2nd of January 1822 at Köslin, in Pomerania. After attending the Gymnasium at Stettin, he studied at Berlin University from 1840 to 1844. In 1848 he took his degree at Halle, and in 1850 was appointed professor of physics in the royal artillery and engineering school at Berlin. Late in the same year he delivered his inaugural lecture as Privatdocent in the university. In 1855 he became an ordinary professor at Zürich Polytechnic, accepting at the same time a professorship in the university of Zürich In 1867 he moved to Würzburg as professor of physics, and two years later was appointed to the same chair at Bonn, where he died on the 24th of August 1888. During the Franco-German War he was at the head of an ambulance corps composed of Bonn students, and received the Iron Cross for the services he rendered at Vionville and Gravelotte. The work of Clausius, who was a mathematical rather than an experimental physicist, was concerned with many of the most abstruse problems of molecular physics. By his restatement of Carnot’s principle he put the theory of heat on a truer and sounder basis, and he deserves the credit of having made thermodynamics a science; he enunciated the second law, in a paper contributed to the Berlin Academy in 1850, in the well-known form, “Heat cannot of itself pass from a colder to a hotter body.” His results he applied to an exhaustive development of the theory of the steam-engine, laying stress in particular on the conception of entropy. The kinetic theory of gases owes much to his labours, Clerk Maxwell calling him its principal founder. It was he who raised it, on the basis of the dynamical theory of heat, to the level of a theory, and he carried out many numerical determinations in connexion with it, e.g. of the mean free path of a molecule. To Clausius also was due an important advance in the theory of electrolysis, and he put forward the idea that molecules in electrolytes are continually interchanging atoms, the electric force not causing, but merely directing, the interchange. This view found little favour until 1887, when it was taken up by S. A. Arrhenius, who made it the basis of the theory of electrolytic dissociation. In addition to many scientific papers he wrote Die Potentialfunktion und das Potential, 1864, and Abhandlungen über die mechanische Wärmetheorie, 1864–1867.

 CLAUSTHAL, or, a town of Germany, in the Prussian Harz, lying on a bleak plateau, 1860 ft. above sea-level, 50. m. by rail W.S.W. of Halberstadt. Pop. (1905) 8565. Clausthal is the chief mining town of the Upper Harz Mountains, and practically forms one town with Zellerfeld, which is separated from it by a small stream, the Zellbach. The streets are broad, opportunity for improvement having been given by fires in 1844 and 1854; the houses are mostly of wood. There are an Evangelical and a Roman Catholic church, and a gymnasium. Clausthal has a famous mining college with a mineralogical museum, and a disused mint. Its chief mines are silver and lead, but it also smelts copper and a little gold. Four or five sanatoria are in the neighbourhood. The museum of the Upper Harz is at Zellerfeld.

Clausthal was founded about the middle of the 12th century in consequence probably of the erection of a Benedictine monastery (closed in 1431), remains of which still exist in Zellerfeld. At the beginning of the 16th century the dukes of Brunswick made a new settlement here, and under their directions the mining, which had been begun by the monks, was carried on more energetically. The first church was built at Clausthal in 1570. In 1864 the control of the mines passed into the hands of the state.

 CLAVECIN, the French for clavisymbal or harpsichord (Ger. Clavicymbel or Dockenklavier), an abbreviation of the Flemish clavisinbal and Ital. clavicimbalo, a keyboard musical instrument in which the strings were plucked by means of a plectrum consisting of a quill mounted upon a jack.

 CLAVICEMBALO, or (from Lat. clavis, key, and cymbalum, cymbal; Eng. clavicymbal, clavisymbal; Flemish, clavisinbal; Span. clavisinbanos), a keyboard musical instrument with strings plucked by means of small quill or leather plectra. “Cymbal” (Gr. , from  , a hollow vessel) was the old European term for the dulcimer, and hence its place in the formation of the word.

 CLAVICHORD, or (Fr. manicorde; Ger. Clavichord; Ital. manicordo; Span. manicordio ), a medieval stringed keyboard instrument, a forerunner of the (q.v.), its strings being set in vibration by a blow from a brass tangent instead of a hammer as in the modern instrument. The clavichord, derived from the dulcimer by the addition of a keyboard, consisted of a rectangular case, with or without legs, often very elaborately ornamented with paintings and gilding. The earliest instruments were small and portable, being placed upon a table or stand. The strings, of finely drawn brass, steel or iron wire, were stretched almost parallel with the keyboard over the narrow belly or soundboard resting on the soundboard bridges, often three in number, and wound as in the piano round wrest or tuning pins set in a block at the right-hand side of the soundboard and attached at the other end to hitch pins. The bridges served to direct the course of the strings and to conduct the sound waves to the soundboard. The scaling, or division of the strings determining their vibrating length, was effected by the position of the tangents. These tangents, small wedge-shaped blades of brass, beaten out at the top, were inserted in the end of the arm of the keys. As the latter were depressed by the fingers the tangents rose to strike the strings and stop them at the proper length from the belly-bridge. Thus the string was set in vibration between the point of impact and the belly-bridge just as long as the key was pressed down. The key being released, the vibrations were instantly stopped by a list of cloth acting as damper and interwoven among the strings behind the line of the tangents.

There were two kinds of clavichords—the fretted or gebunden and the fret-free or bund-frei. The term “fretted” was applied to those clavichords which, instead of being provided with a string or set of strings in unison for each note, had one set of strings acting for three or four notes, the arms of the keys being twisted in order to bring the contact of the tangent into the acoustically correct position under the string. The “fret-free” were chromatically-scaled instruments. The first bund-frei clavichord is attributed to Daniel Faber of Crailsheim in Saxony about 1720. This important change in construction increased the size of the instrument, each pair of unison strings requiring a key and tangent of its own, and led to the introduction of the system of tuning by equal temperament upheld by J. S. Bach. Clavichords were made with pedals.

The tone of the clavichord, extremely sweet and delicate, was characterized by a tremulous hesitancy, which formed its great charm while rendering it suitable only for the private music room or study. Between 1883 and 1893 renewed attention was drawn to the instrument by A. J. Hipkins’s lectures and recitals on keyboard instruments in London, Oxford and Cambridge; and Arnold Dolmetsch reintroduced the art of making clavichords in 1894.

 CLAVICYTHERIUM, a name usually applied to an upright (q.v.), the soundboard and strings of which were vertical instead of horizontal, being thus perpendicular to the keyboard; but it would seem that the clavicytherium proper is distinct from the upright spinet in that its strings are placed horizontally. In the early clavicytherium there was, as in the spinet, only one string (of gut) to each key, set in vibration by means of a small quill or leather plectrum mounted on a jack which acted as in the spinet and (q.v.). The clavicytherium or keyed