lit·mus 
(lĭtməs)
(lĭtməs)n.
A water-soluble bluish powder derived from certain lichens that changes to red with increasing acidity and to deeper blue with increasing basicity.
[Middle English litemose (of Scandinavian origin; akin to Old Norse litmosi, dyer's herbs litr, color, dye + mosi, bog, moss) and Middle English lykemose (from Middle Dutch lijkmoes, variant of lēcmoes : lēken, to drip + moes, moss).]