What is "bitter salt"

Bitter \Bit"ter\, a. [AS. biter; akin to Goth. baitrs, Icel. bitr, Dan., Sw., D., & G. bitter, OS. bittar, fr. root of E. bite. See Bite, v. t.]

1. Having a peculiar, acrid, biting taste, like that of wormwood or an infusion of hops; as, a bitter medicine; bitter as aloes.

2. Causing pain or smart; piercing; painful; sharp; severe; as, a bitter cold day.

3. Causing, or fitted to cause, pain or distress to the mind; calamitous; poignant.

   It is an evil thing and bitter, that thou hast forsaken the Lord thy God.
   --Jer. ii. 19.

4. Characterized by sharpness, severity, or cruelty; harsh; stern; virulent; as, bitter reproach.

   Husbands, love your wives, and be not bitter against them.
   --Col. iii. 19.

5. Mournful; sad; distressing; painful; pitiable.

   The Egyptians . . . made their lives bitter with hard bondage.
   --Ex. i. 14.

   Bitter apple, Bitter cucumber, Bitter gourd. (Bot.) See Colocynth.

   Bitter cress (Bot.), a plant of the genus Cardamine, esp. Cardamine amara.

   Bitter earth (Min.), tale earth; calcined magnesia.

   Bitter principles (Chem.), a class of substances, extracted from vegetable products, having strong bitter taste but with no sharply defined chemical characteristics.

   Bitter salt, Epsom salts; magnesium sulphate.

   Bitter vetch (Bot.), a name given to two European leguminous herbs, Vicia Orobus and Ervum Ervilia.

   To the bitter end, to the last extremity, however calamitous.

   Syn: Acrid; sharp; harsh; pungent; stinging; cutting; severe; acrimonious.