114 FOO TNO TES FKOM 



pended to some from the bare, desolate Gibraltar of the 

 Eed Sea, the rock of Aden in Arabia, stated most sug- 

 gestively "Abundant, but unknown as an article of 

 commerce." It is probable that it occurs on the mari- 

 time rocks of all tropical countries in equal profusion. 

 In appearance this valuable lichen resembles a diminutive 

 leafless shrub, forked, and subdivided into numerous 

 roundish, irregular branches. It is tough and leathery in 

 texture, of a whitish or blue grey colour, and covered 

 with a mealy powder, or scattered warty excrescences. 

 It is imported in the same state in which it was gathered 

 from the volcanic rocks ; and those who prepare it for 

 the use of the dyer grind it between stones, so as 

 thoroughly to bruise but not to reduce it to powder, 

 moistening it occasionally with ammonia mixed with 

 quick-lime. By this process it acquires in a few days a 

 purplish-red tinge, and is found to form a confused mass 

 of violet-coloured threads. In this state it is employed 

 to give the English broadcloths that peculiar lustre and 

 purple tint, when viewed in a certain light, which are 

 so much admired. When beaten to a pulp, and dried in 

 little cubes about the size of dice, which .have an azure 

 colour with white spots, and an unpleasant odour, the 

 orchil is called litmus. This substance contains, accord- 

 ing to Gelis, three colouring principles : one soluble in 

 ether, which is orange-red ; one soluble in alcohol, and 

 one in water, both of which have a most beautiful pur- 

 ple tint, which they lose when excluded from the air, 

 and regain when again exposed. On account of its ex- 

 ceeding delicacy, and the ease with which it may be 

 applied, litmus is chemically used as a test of akaliuity 



