Page:The New International Encyclopædia 1st ed. v. 01.djvu/117

ACHROMATISM. bluish-grpen, i.e. tlu'^e twii culois are brought t(i tlw saiiio fdoiis; Imt if it is to Ix' used for photo<jraphic purposes, it is "oorrected" for two wave-lenstlis, which incliide lliose radiations pos- scssiiij,' the greatest pliotogiaptiic action. There are two defects whieli a lens may liave, owing to chromatic aberration (q.v.), in tluit the colored images may be at different distances from the lens and that they may be of ditVerent sizes. The second of these defects is in.signiticant if the lens is thin; and the first may be 'ciirrei'tcd," as just described, by combining two tliin li'nses. If the lens is thick, or if the lenses of the lens-system are some ilistance apart, the second of the above mentioned errors becomes serious. It may, how- ever, he corrected.

ACHTERMANN, iic'ter-mAn, Theouore Wii.iiKi.M I ITD'.i - KSS4). A (Jerman sculptor. In his sculptures he devoted himself principally to New Testament subjects. While at Rome, in 1841. he prepared a statue of Christ and an "Ecce Homo" for the Duke of Aremberg. His most celebrated luoductions are preserved in the cathedral at lliinster, and consist of a "Pietil" and a "Descent from the Cross." Another ad- mirable work is the marble altar on which are de- picted three episodes from the life of Christ ( in relief), prepared in I.S73 for the cathedral at Prague.

A CHULA, a-sho75'la (Portug.). A dance similar to the fandango (q.v.).

ACHURCH', Janet. The stage name of Janet Achurch Sharp, an English actress, the wife of Mr. Charles Charrington. She was born in Lancashire and first appeared in London at the Olympic Theatre in January, 1883. In 1887 she joined Beerbohm Tree's company, and at the Novelty Theatre, June 7, 188!), created in English the part of Nora Helmer in A Doll's House. This was the first presentation of an Ibsen play to the English public. She has since toured with a (ompany in India and Australia, and appeared in the United States with Richard Mansfield (18n,5), and independently. In June, 1897, at the Olympic Theatre, London, she took the Shakespearean part of Cleopatra to the Antony of Louis Calvert.

ACHZIB, flk'zib. (1). A Phoenician city claimed by Asher (Joshua xix:29), but not conquered (Judges i:IU); the modern Ez-Zib on the promontory of Ras-en-Nakurah. Aehzib is mentioned by Sennacherib. (2) A town in the Shephelah of .Tudah (Joshua xv: 44). Possibly the modern 'Ain-el-Kezbeh. near Bet-Nettif.

ACIDASPIS,!ls'r-(lris'])rs (Gk. aKi(, akis, spine -|- (jaTr/f, (/.<i/)i.s, sliicld). A peculiar genus of triloliites foiiiul in rocks of Silurian and Devonian age in nearly all parts of the world. The individuals arc. as a rule, small, and are .remarkable becau.sc of the spiny ornamenta- tion of the dorsal shield or carapace. The loba- tion of the head shield is rather peculiar and quite unlike that seen in any other genus of trilobites, the trilobite divisicm being obscured by a number of supplementary furrows and by the strong development of two longitudinal false furrows between the normal dorsal furrows. The thorax contains nine or ten segments, and the tail-shield is of rather small size. In some species a row of slender spines is developed upon the sides of the head-shield and a long spine projects from each posterior angle. Besides these there are often two long straight or curved spines directed upward and backward from the middle posterior edge of the head. Each .seg- ment of the thorax is produced laterally into long spines, and there are also two short spines on the raised median portion of each segment. The tail-shield is in nearly all species likewise furnished with spines, so that on the whole these animals must, though of small size, have presented a rather formidable aspect to larger animals which sought to prey upon them. A few species of the genus are of particular interest on account of the abnormal development of the eyes, which are placed at the summits of highly elevated slender, though immovable, stalks, which arrangement enabled the animal to com- mand a view in all directions. This elevation of the eye recalls the stalk-eyes of some nifidern crabs and lobsters. For illustration, see Plate of Tbilobites.

ACIDIMETRY, as'I-dlm'^-trl (Lat. acidus, sour + tik. /lirpdi; metrnn, measure). The determination of the amount of acid contained in a solid or liquid substance. When the cnm- ])ound is a solid, the determination is usually made by the gravimetric method, which consists in the dissolving of a known weight of the material, and its sub.sequent treatment by such reagents as will yield an insoluble compound, from the weight of which the amount of acid can be calculated. When the substance is a liquid, free Irom foreign matter, the proportion of acid may be ascertained by determining the specific gravity of the solution by means of a hydrometer, but in case of mixtures the acidity of a solution is best ascertained by the volu- metric method, which is described under Alkali- METEU.

ACIDS, fisldz (Lat. acidus, sour). A large and important class of chemical substances. They all contain hydrogen, part or all of which is replaced by metals when the acids are brought in contact with metallic hydroxides. The com- pounds formed by substituting metals for the hydrogen of acids are termed the salts of those metals, and therefore the acids themselves may be regarded as salts of hydrogen. An example inav render these definitions more clearly intel- ligible. When the sour principle of vinegar is brought in contact with potassium hydroxide, a reaction ensues, resulting in the formation of a new substance. A chemical analysis, com- bined with a determination of the molecular weight of the sour principle of vinegar, shows that the molecule of the latter must be repre- sented by the formula CJI.O,: on the other hand, the substance fiu-mcd with ]iotassium hydroxide is represented by the formula CJl.KO,. Evidently, part of the hydrogen of the sour prin- ciple of vinegar has been replaced by the metal principle of vinegar with the acids (it is the well- known acetic acid): and wr class the substance obtained by its action on potassium hydroxide with the salts (it is called the acetate of potas- sium, while acetic acid itself may be called the acetate of hydrogen).
 * iotassium ( K ) . We tlieref(ue class the sour

Most acids have a sour taste and change the blue color of litmus to red. These properties, however, are not strictly characteristic of acids, silicic acid, for instance, possessing neither, thoush — like a true acid — it combines with metallic hydroxides to form salts.