Lit′mus or Lacmus, a coloring matter manufactured in HollandLichens (Rocella tinctoria and others related to it) are reduced to a pulp with water, and potassium carbonate and ammonia are added.  The mass gradually assumes a blue color, due to some attribute of the lichen.  Chalk or gypsum is added to render the mass thick enough to be formed into rectangular cakes, which when dried are ready for use.  Litmus is never used as a dye, but by chemists to detect free acids and free alkalies.  The blue of litmus is turned to red by an acid, and the color again becomes blue by being mixed with an alkali.  Litmus paper, i. e., paper infused with litmus both in its blue and red state, is the form in which litmus is generally used as a test.