TECHNICAL INDEX AND GLOSSARY 



(Under this alphabet, technical and semitechnical 

 terms are defined, with illustrative comment, and often 

 with the addition of historical data. The references to 

 the text, by volume and page, though specific, are in- 

 tended rather as general guides to the reader than as 

 offering a complete or detailed analysis of the subject. 



For a more detailed analysis see the Encyclopaedic 

 General Index in this volume. For biographical data 

 see the Biographical Index.) 



Aberration, (i) In physics, that property of a lens in virtue 

 of which it forms an image with color fringes, due to the fact 

 that different rays of light are not brought precisely to the 

 same focus. This difficulty is never entirely overcome even 

 with the finest astronomical lenses. (2) In astronomy, the dis- 

 placement of the apparent position of a celestial body due to 

 the fact that the velocity of light is not infinite. See "Bradley 

 and the Aberration of Light," Vol. Ill, p. II. 



Abrasives. Those substances used in grinding and polishing. 

 See "Gems, Natural and Artificial," Vol. IX, p. 295; in particular 

 p. 306; also "Glass and Glass Making," Vol. IX, p. 293. 



Absolute Zero. The hypothetical condition of matter at which 

 its molecules or atoms are in such a state of quiescence that 

 they give out no heat. Theoretically this point lies 272 below 

 the Centigrade zero; practically it has not been demonstrated 

 iit the laboratory. See "The Royal Institution and the Low 

 Temperature Researches," Vol. V, Chapter 3; in particular p. 69. 



Acetylene. A hydrocarbon gas, made commercially by adding 

 water to calcium carbide; it has the chemical formula C S H 2 . See 

 "The Introduction of Acetylene Gas," Vol. VI, p. 212. 



Actinium. An element occurring in nature associated with 

 zinc. Discovered in 1881 by Dr. T. L. Phipson. See "Element" 

 and "Periodic Law," under present alphabet. 



Aeroplane. The apparatus, consisting originally of a canvas 

 plane or planes supported by poles and wires and actuated by 

 motor-driven propellers, which constituted the first heavier- 

 than-air machine in which human flight was accomplished. The 



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