Seidlitz powder is the name with which is commonly known a medication composed by a mixture of tartaric acid, sodium bicarbonate, and potassium sodium tartrate, used as a mild cathartic by dissolving in water and drinking.
After ingestion, the powder combines with gastric juices developing intestinal gases which are somewhat helpful in evacuating the taker's bowels. However their use could also lead to unpleasant side effects, and could be even fatal in subjects with conditions such as hernia, bowel obstructions and other ailments.
This medication's name comes from the Seidlitz Saline Springs of Bohemia (now Sedlčany in the Czech Republic), which were rather famous in Europe at the time this medication was first marketed in the late 1800s, even though the foregoing laxative constituents do not represent those of the springs named.

References

• webpage with instructions on how to prepare the powder