LICHENS. 175 
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, according 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 purple 
tint, which they lose when excluded from the air, 
and regain when again exposed. On account of 
its exceeding delicacy, and the ease with which it 
may be applied, litmus is chemically used as a 
test of akalinity and acidity in the form of paper 
saturated with it, preserved in well-closed vessels, 
and secluded from the influence of light. This 
paper is turned red by an acid, and is restored to 
its original blue colour by an alkali. Orchil 
contains certain other substances, called orcine 
and erythrine, which are perfectly colourless, and 
contain no nitrogen; but when exposed to the 
action of ammonia and common atmospheric air, 
they yield exquisitely beautiful colouring matters, 
which crystallize in regular flat quadrangular 
prisms, have a very sweet flavour, and of which 
nitrogen is an essential element. 
In the Canary and Cape de Verde Islands, 
orchil was at one time the most important 
article of commerce; the annual exportation 
