The Collaborative International Dictionary of English v.0.48:

Seidlitz \Seid"litz\, a.
   Of or pertaining to Seidlitz, a village in Bohemia. [Written
   also Sedlitz.]
   [1913 Webster]

   Seidlitz powders, effervescing salts, consisting of two
      separate powders, one of which contains forty grains of
      sodium bicarbonate mixed with two drachms of Rochell? salt
      (tartrate of potassium and sodium) and the other contains
      thirty-five grains of tartaric acid. The powders are mixed
      in water, and drunk while effervescing, as a mild
      cathartic; -- so called from the resemblance to the
      natural water of Seidlitz. Called also Rochelle powders.


   Seidlitz water, a natural water from Seidlitz, containing
      magnesium, sodium, calcium, and potassium sulphates, with
      calcium carbonate and a little magnesium chloride. It is
      used as an aperient.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Seidlitz powders
    n 1: an effervescing salt containing sodium bicarbonate and
         Rochelle salt and tartaric acid; used as a cathartic [syn:
         Seidlitz powder, Seidlitz powders, Rochelle powder]