The Collaborative International Dictionary of English v.0.48:

Beta rays \Be"ta rays\ (Physics)
   a form of ionizing radiation emitted by radioactive
   substances (such as radium), more penetrating than alpha
   rays, and consisting of negatively charged electrons. The
   electrons are the same kind of particle as those of cathode
   rays, but have much higher velocities (about 35,000 to
   180,000 miles per second). They are readily deflected by a
   magnetic or electric field.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Electron \E*lec"tron\, n. [NL., fr. Gr. 'h`lektron. See
   Electric.]
   1. Amber; also, the alloy of gold and silver, called
      electrum. [archaic]
      [1913 Webster]

   2. (Physics & Chem.) one of the fundamental subatomic
      particles, having a negative charge and about one
      thousandth the mass of a hydrogen atom. The electron
      carries (or is) a natural unit of negative electricity,
      equal to 3.4 x 10^-10 electrostatic units, and is
      classed by physicists as a lepton. Its mass is
      practically constant at the lesser speeds, but increases
      due to relativistic effects as the velocity approaches
      that of light. Electrons are all of one kind, so far as is
      known. Thus far, no structure has been detected within an
      electron, and it is probably one of the ultimate composite
      constituents of all matter. An atom or group of atoms from
      which an electron has been detached has a positive charge
      and is called a cation. Electrons are projected from the
      cathode of vacuum tubes (including television picture
      tubes) as cathode rays and from radioactive substances
      as the beta rays. Previously also referred to as
      corpuscle, an obsolete term. The motion of electrons
      through metallic conductors is observed as an electric
      current. A particle identical to the electron in mass and
      most other respects, but having a positive instead of a
      negative charge, is called a positron, or antielectron
      [Webster 1913 Suppl. +PJC] Electro-negative