TECHNICAL INDEX AND GLOSSARY 



covered, or at all events introduced to the Western world and 

 named by mediaeval Arabian physicians. See "Mediaeval Science 

 Among the Arabians," Vol. II, p. 22. 



Alizarine. A substance formerly obtained from the root of the 

 madder plant, Rubia tinctorum, but now chiefly produced from 

 coal-tar. It has great commercial importance as a dye, produc- 

 ing the color known as "turkey red." The growth of the madder 

 plant was formerly an important industry, but like the cultivation 

 of the indigo plant, this pursuit has been made superfluous by 

 the cheapness of the coal-tar colors. See "Pigments from Vege- 

 table and Animal Sources," Vol. VIII, p. 302; in particular pp. 

 307-309. 



Alloy. An intimate homogeneous mixture of different metals, 

 usually produced by fusion. Thus brass is an alloy of copper 

 and zinc; pewter, an alloy of tin and lead; steel, an alloy of iron 

 and various other elements, chief among which are carbon, 

 nickel, manganese, and tungsten. See chapter "The Age of 

 Steel," Vol. VI, p. 271; in particular "The Conversion of Iron 

 Ore into Iron and Steel," p. 283, and "Alloy Steels," p. 295. 



Alternating current. The electrical current produced by a 

 dynamo not provided with a commutator. See "The Mechanism 

 of the Dynamo," Vol. VI, p. 173. 



Aluminum. A nearly white metal, about as hard as silver, 

 and having a tensile strength about equal to that of copper, but 

 many times lighter in weight than either. It alloys with almost 

 all the metals, and for this reason, and because of its lightness, is 

 of great commercial importance. Aluminum oxides are the base 

 of the important corundum group of gems, including true rubies, 

 sapphires, and emeralds. See "The Ruby and Its Allies," Vol. 

 IX, p. 319; also "Artificial Gems," Vol. IX, p. 331. Though 

 aluminum is a chief constituent of clayey soils, its isolation has 

 been difficult. For the electrolytic method of its production, see 

 "Some Recent Triumphs of Applied Science," Vol. VI, p. 300. 



Amethyst. A variety of corundum (a crystalline oxide of 

 aluminum), its violet or purple color being due probably to 

 traces of manganese or of iron. See "The Ruby and its Allies," 

 Vol. IX, p. 319. 



Ammonia. A gaseous compound of hydrogen and nitrogen, 

 having the formula N H 3 . It is supposed to derive its name from 

 the fact that it was originally prepared near the temple of 

 Ammon, in Egypt. Its composition was, of course, unknown 

 until the time of the "pneumatic chemists" (including Black, 



