LICHENS 119 



by treating the Lichen Lecanora tartar ea, which grows 

 on rocks, with stale urine, water, and chalk. Litmus, 

 which is exceedingly valuable to chemists as a test for 

 acids or alkalies, is prepared from certain Lichens. 

 An acid turns blue litmus red; an alkali, such as am- 

 monia, will change the red back to blue. 



