TECHNICAL INDEX AND GLOSSARY 



Undulatory Theory of Light. According to this theory light 

 is a kind of undulatory motion produced by the luminous body in 

 the particles of an elastic, imponderable medium called the 

 luminiferous ether, which is supposed to fill all space, and also 

 the interstices of all bodies. See "Thomas Young and the Wave 

 Theory of Light," Vol. Ill, p. 215. 



Uranium. A metallic chemical element discovered by Klap- 

 roth in 1789, and first isolated by Peligot in 1842. Its atomic 

 weight is 238.5, specific gravity 18.6, symbol U. 



Valence (valency). The combination value or capacity of a 

 chemical atom, in virtue of which it can unite with one only or 

 with more than one atom equivalent to the hydrogen atom. 

 See "Chemical Affinity," Vol. IV, p. 57. 



Valves of the Veins. Structures in the lumen of veins which 

 prevent the flow of blood backward away from the heart. They 

 were discovered and described by the French anatomist, Charles 

 Etienne (1503-1564). See Vol. II, p. 166. 



Varnish. A solution of certain resins, such as mastic, lac, 

 copel, asphalt, amber, benzoin, etc., capable of hardening with- 

 out losing its transparency. Ordinary commercial varnish is a 

 solution of resin in oil of turpentine. See "Varnishes," Vol. 

 VIII, p. 316. 



Vitagraph. A moving-picture machine similar to the kinet- 

 oscope, patented by Thos. A. Edison in 1891. See "Chrono- 

 photography Moving Pictures," Vol. VIII, p. 248. 



Vitalists. Followers of a system of medicine championed by 

 Paul Joseph Barthez in the eighteenth century. They assumed 

 that there was a "vital principle," of unknown nature, but differ- 

 ing from the thinking mind, or the soul, which was the cause of 

 all the phenomena of life. See "Animists, Vitalists, and Or- 

 ganicists," Vol. IV, p. 184. 



Vitascope. See "Vitagraph" of the present index. 



Volt. The unit of electro-motive force. See "Electricity" of 

 the present index. 



Vortex Atom. See "Vortex Theory." 



Vortex Theory. A conception that the atoms and molecules 

 of physical science may be vortex rings or filaments, or com- 

 binations of these, in the universal ether. See "Physical Prob- 

 lems," Vol. V, p. 213. 



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