_Vitalism._ The theory that there is a special force in living beings, distinct from the ordinary physical and chemical forces. Few scientists are vitalists, although many preachers are, and a number of philosophers as well. _Vitamines._ Organic substances found in various foods and apparently necessary to human life. Their exact chemical constitution remains unknown. _Volt._ The unit of electromotive force. The pressure which produces a current of one ampere when applied to a conductor with a resistance of one ohm. _Volume._ The space a body occupies. _Wave._ A disturbance of the particles of a fluid medium as in the transmission of sound, heat, and light. _Weight._ The force with which the earth attracts a body. See Mass. _Work._ The expenditure of force in overcoming force or producing a molecular change. Futile efforts do not constitute work. _Xenon._ A gaseous element. Symbol: X. _X-Ray._ See Röntgen ray. _Xylogy._ The study of the structure of wood. _Yttrium._ A metallic element. Symbol: Y or Yt. _Zinc._ A metallic element. Symbol: Zn. _Zirconium._ A metallic element. Symbol: Zr. _Zoöchemistry._ The study of the constituents of animal bodies. _Zoölogy._ The science of animals. _Zygote._ A cell formed by the joining of a male and a female cell, capable of developing into a new individual. _Zymosis._ Fermentation. _Zymurgy._ The chemistry of fermentation processes. Transcriber’s Notes: Italics are shown thus: _sloping_. Variations in spelling and hyphenation are retained. Perceived typographical errors have been changed.