The Collaborative International Dictionary of English v.0.48:

Ratsbane \Rats"bane`\ (r[a^]ts"b[=a]n`), n. [Rat + bane.]
   Rat poison; white arsenic.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arsenic \Ar"se*nic\ ([aum]r"s[-e]*n[i^]k; 277), n. [L.
   arsenicum, Gr. 'arseniko`n, 'arreniko`n, yellow orpiment,
   perh. fr. 'arseniko`s or better Attic 'arreniko`s masculine,
   'a`rrhn male, on account of its strength, or fr. Per.
   zern[imac]kh: cf. F. arsenic.]
   1. (Chem.) One of the elements, a solid substance resembling
      a metal in its physical properties, but in its chemical
      relations ranking with the nonmetals. It is of a
      steel-gray color and brilliant luster, though usually dull
      from tarnish. It is very brittle, and sublimes at 356[deg]
      Fahrenheit. It is sometimes found native, but usually
      combined with silver, cobalt, nickel, iron, antimony, or
      sulphur. Orpiment and realgar are two of its sulphur
      compounds, the first of which is the true arsenicum of the
      ancients. The element and its compounds are active
      poisons. Specific gravity from 5.7 to 5.9. Atomic weight
      75. Symbol As.
      [1913 Webster]

   2. (Com.) Arsenious oxide or arsenious anhydride; -- called
      also arsenious acid, white arsenic, and ratsbane.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ratsbane
    n 1: a white powdered poisonous trioxide of arsenic; used in
         manufacturing glass and as a pesticide (rat poison) and
         weed killer [syn: arsenic, arsenic trioxide, arsenous
         anhydride, arsenous oxide, white arsenic, ratsbane]