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29S
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GLOSSARY.
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newer has been deposited on the eroded surface of the
older. Undulations of Light.—The waves in the ether which give
rise to the phenomenon of light. Uniaxial Crystals.—Those having one direction, and one
only, in which light suffers no double refraction ; crystals
belonging to the tetragonal and hexagonal systems are
uniaxial. Unit Rhojibohedron.—The rhombohedron which may be
regarded as developed from the extension of alternate faces
of a hexagonal pyramid, each face of which pyramid would
cut the vertical axis at unit length and two of the horizontal
axes at unit length. Vacuum Tube.—A closed tube which contains two electrical
terminals and which has been exhausted to a high degree ;
on passing a current of electricity of high potential through
the tube, emanations, whose properties differ widely from
ordinary light waves, are produced. Vibrations of Light.—See " Undulations." Zeolite.—A Zeolite ma}~ be regarded as a mineral consisting of
hydrous non-magnesian silicates, of epigene origin, which
froths up before the blow-pipe.
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