TECHNICAL INDEX AND GLOSSARY 



Gears. Mechanisms for transmitting power, or for changing 

 the direction of its action. The toothed wheel and the bitted 

 wheel are typical examples. See "Other Means of Transmitting 

 Power," Vol. VI, p. 35. 



Gems. Minerals, usually in crystalline form, the combined 

 beauty and rarity of which gives them great commercial value, 

 usually quite without regard to any question of practical utility. 

 The diamond (a crystal of pure carbon) owes its popularity to its 

 hardness, combined with its high index of refraction; the true 

 ruby, sapphire, and emerald are crystallized oxides of aluminum 

 (corundum) with traces of different coloring matters. Stones 

 of the corundum order are now duplicated in the laboratory on 

 a commercial scale; but laboratory diamonds are as yet of in- 

 finitesimal size. See "Gems, Natural and Artificial," Vol. IX, 

 p. 295. 



Geocentric Theory. The theory according to which the earth 

 is the center of the solar system. This theory has universal 

 vogue (notwithstanding the protest of Aristarchus) throughout 

 antiquity; the true heliocentric theory was put forward by 

 Copernicus. See "Aristarchus of Samos, the Copernicus of 

 Antiquity," Vol. I, p. 212. 



Geology. The science that deals with the origin, structure, 

 and metamorphoses of the earth's crust. It is, in any compre- 

 hensive sense, a modern science, its correct theories dating from 

 the close of the eighteenth century. See "The Origin and De- 

 velopment of Modern Geology," Vol. Ill, p. 116. 



Geometry. Literally, and in its earliest applications, the 

 science of earth measurement. As such its elements were un- 

 derstood by the ancient Egyptians. Expanded and its chief 

 propositions formulated by the Alexandrian, Euclid (300 B.c), 

 whose exposition is still used in modern text-books. See 

 "Euclid," Vol. I, p. 192. 



Germanium. See "Elements" in the present index. 



Glands. Secretory organs, of which there are many types, as, 

 for example, the salivary glands, the peptic glands of the 

 stomach, the pancreatic glands, and sundry intestinal glands. 

 See also "Ductless Glands" of the present index. 



Glass. A crystalline substance, the chief constituent of which 

 is silica (sand), with which varying proportions of potash, soda, 

 and lime are combined. See "Glass and Glass-Making," Vol. IX, 

 p. 277. 



Glass Negatives. See "Photography" in the present index. 



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