Alpha rays. (Physics & Chem.) Rays of relatively low penetrating power emitted by radium and other radioactive substances, and shown to consist of positively charged particles (perhaps particles of helium) having enormous velocities but small masses. They are slightly deflected by a strong magnetic or electric field.
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Webster's Dictionary 1913
Webster's Revised Unabridged Dictionary, C. & G. Merriam Co., 1913.