KEY AND INDEX 



this character are always of interest, but are no longer startling 

 as they were at the middle of the nineteenth century, when the 

 "age of man" was supposed to compass a mere six thousand 

 years. See "The New Science of Paleontology," Vol. Ill, p. 98. 

 Friction. The motion-retarding influence exerted by one sur- 

 face against another, or by a gas or liquid acting on another gas 

 or liquid or on a solid. Friction is responsible for enormous loss 

 of power in the operation of all working mechanisms; but, on 

 the other hand, were it not for friction man would not be able 

 to handle tools, or even to walk. See "Other Means of Trans- 

 mitting Power," Vol. VI, p. 35. 



Ganglion Cells. Central nerve cells, located in the brain, the 

 spinal cord, or in the outlying plexuses, which serve as store- 

 houses of nervous energy. See "Functions of the Nerves," Vol. 

 IV, p. 259. 



Gas. Matter in the state of tennosity in which its component 

 molecules, instead of being relatively close together and more 

 or less subject to the influence of cohesive forces (solids, 

 liquids), are widely separated, and in rapid linear motion, dart- 

 ing hither and thither, and rebounding on contact. (See Max- 

 well; Kinetic theory.) All kinds of matter assume the gaseous 

 state under proper conditions of pressure and temperature. 

 Substances that are gaseous at ordinary terrestrial temperatures 

 are spoken of as "permanent gases," but the term is a mis- 

 nomer. For Clerk-Maxwell's kinetic theory of gases, see Vol. 

 Ill, p. 295. For the "Law of Avogadro," see Vol. IV, p. 57. 

 For recent experiments in liquifying gases, see Vol. V, p. 53. 

 For some recent discoveries of new gases, see Vol. V, p. 84. 



Gas Engine. An engine actuated by some gas other than 

 steam. Gas engines are of recent development; the best-known 

 form being the explosion engine generally used for automobiles 

 and motor boats. See "Gas and Oil Engines," Vol. VI, p. 132. 



Gas Mantle. A mantle, of which that of Herr Welsbach is 

 the type, composed of an earth (e.g., a compound of thorium 

 and cerium) which becomes incandescent when heated in a gas 

 flame; giving far more light than would be produced by the gas 

 flame itself. See "The Incandescent Gas Mantle," Vol. VI, p. 208. 



Gastric Juice. The digestive juice secreted by the stomach. 

 It is acidulous (hydrochloric acid) and its most characteristic 

 and important constituent is the ferment pepsin. See "The 

 Chemical Theory of Digestion," Vol. IV, p. 88. 



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