CAUSEWAY, a path on a raised dam or mound across marshes or low-lying ground; the word is also used of old paved highways, such as the Roman military roads. “Causey” is still used dialectically in England for a paved or cobbled footpath. The word is properly “causey-way,” from causey, a mound or dam, which is derived, through the Norman-French caucie (cf. modern chaussée), from the late Latin via calciata, a road stamped firm with the feet (calcare, to tread).
CAUSSES (from Lat. calx through local Fr. caous, meaning “lime”), the name given to the table-lands lying to the south of the central plateau of France and sloping westward from the Cévennes. They form parts of the departments of Lozère, Aveyron, Card, Hérault, Lot and Tarn-et-Garonne. They are of limestone formation, dry, sterile and treeless. These characteristics are most marked in the east of the region, where the Causse de Sauveterre, the Causse Méjan, the Causse Noir and the Larzac flank the Cévennes. Here the Causse Méjan, the most deserted and arid of all, reaches an altitude of nearly 4200 ft. Towards the west the lesser causses of Rouergue and Quercy attain respectively 2950 ft. and 1470 ft. Once an uninterrupted table-land, the causses are now isolated from one another by deep rifts through which run the Tarn, the Dourbie, the Jonte and other rivers. The summits are destitute of running water, since the rain as it falls either sinks through the permeable surface soil or runs into the fissures and chasms, some of great depth, which are peculiar to the region. The inhabitants (Caussenards) of the higher causses cultivate hollows in the ground which are protected from the violent winds, and the scanty herbage permits of the raising of sheep, from the milk of which Roquefort cheeses are made. In the west, where the rigours of the weather are less severe, agriculture is more easily carried on.
CAUSSIN DE PERCEVAL, ARMAND-PIERRE (1795-1871), French orientalist, was born in Paris on the 13th of January 1795. His father, Jean Jacques Antoine Caussin de Perceval (1759-1835), was professor of Arabic in the Collège de France. In 1814 he went to Constantinople as a student interpreter, and afterwards travelled in Asiatic Turkey, spending a year with the Maronites in the Lebanon, and finally becoming dragoman at Aleppo. Returning to Paris, he became professor of vulgar Arabic in the school of living Oriental languages in 1821, and also professor of Arabic in the College de France in 1833. In 1849 he was elected to the Academy of Inscriptions. He died at Paris during the siege on the 15th of January 1871.
Caussin de Perceval published (1828) a useful Grammaire arabe vulgaire, which passed through several editions (4th ed., 1858), and edited and enlarged Élie Bocthor’s[1] Dictionnaire français-arabe (2 vols., 1828; 3rd ed., 1864); but his great reputation rests almost entirely on one book, the Essai sur l’histoire des Arabes avant l’Islamisme, pendant l’époque de Mahomet (3 vols., 1847-1849), in which the native traditions as to the early history of the Arabs, down to the death of Mahommed and the complete subjection of all the tribes to Islam, are brought together with wonderful industry and set forth with much learning and lucidity. One of the principal MS. sources used is the great Kitáb al-Agháni (Book of Songs) of Abu Faraj, which has since been published (20 vols., Boulak, 1868) in Egypt; but no publication of texts can deprive the Essai, which is now very rare, of its value as a trustworthy guide through a tangled mass of tradition.
CAUSTIC (Gr. καυστικός, burning), that which burns. In surgery, the term is given to substances used to destroy living tissues and so inhibit the action of organic poisons, as in bites, malignant disease and gangrenous processes. Such substances are silver nitrate (lunar caustic), the caustic alkalis (potassium and sodium hydrates), zinc chloride, an acid solution of mercuric nitrate, and pure carbolic acid. In mathematics, the “caustic surfaces” of a given surface are the envelopes of the normals to the surface, or the loci of its centres of principal curvature.
In optics, the term caustic is given to the envelope of luminous rays after reflection or refraction; in the first case the envelope is termed a catacaustic, in the second a diacaustic. Catacaustics are to be observed as bright curves when light is allowed to fall upon a polished riband of steel, such as a watch-spring, placed on a table, and by varying the form of the spring and moving the source of light, a variety of patterns may be obtained. The investigation of caustics, being based on the assumption of the rectilinear propagation of light, and the validity of the experimental laws of reflection and refraction, is essentially of a geometrical nature, and as such it attracted the attention of the mathematicians of the 17th and succeeding centuries, more notably John Bernoulli, G.F. de l’Hôpital, E.W. Tschirnhausen and Louis Carré.
The simplest case of a caustic curve is when the reflecting surface is a circle, and the luminous rays emanate from a point on the circumference. If in fig. 1 AQP be the reflecting circle having C as centre, P the luminous point, and PQ any Caustics by reflection. incident ray, and we join CQ it follows, by the law of the equality of the angles of incidence and reflection, that the reflected ray QR is such that the angles RQC and CQP are equal; to determine the caustic, it is necessary to determine the envelope of this line. This may be readily accomplished geometrically or analytically, and it will be found that the envelope is a cardioid (q.v.), i.e. an epicycloid in which the radii of the fixed and rolling circles are equal. When the rays are parallel, the reflecting surface remaining circular, the question can be similarly treated, and it is found that the caustic is an epicycloid in which the radius of the fixed circle is twice that of the rolling circle (fig. 2). The geometrical method is also applicable when it is required to determine the caustic after any number of reflections at a spherical surface of rays, which are either parallel or diverge from a point on the circumference. In both cases the curves are epicycloids; in the first case the radii of the rolling and the fixed circles are a(2n − 1)/4n and a/2n, and in the second, an/(2n + 1) and a/(2n + 1), where a is the radius of the mirror and n the number of reflections.