THE PAGE OF NATURE. 115 



and acidity in the form of paper saturated with it, pre- 

 served in well-closed vessels, and secluded from the in- 

 fluence of light. This paper is turned red by an acid, 

 and is restored to its original blue colour by an alkali. 

 The orchil contains certain other substances, called orcine 

 and erythrine, which are perfectly colourless, and contain 

 no nitrogen ; but when exposed to the action of ammo- 

 nia 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, the orchil was at 

 one time the most important article of commerce ; the 

 annual exportation being valued at from 60,000 to 

 80,000 ; but so great has been its consumption of 

 late years, that the best quality, which generally sells 

 for 200 a ton, and has in times of scarcity been ac- 

 tually sold for the enormous sum of 1000, or about 

 9s. a pound, has become exceedingly rare, and what is 

 now commonly imported from other countries is worth 

 little more than 30 the ton. 



In this country there are many species of lichens, 

 growing in greater or less abundance, on the mountain 

 rocks, which might be advantageously substituted for 

 the rare and expensive foreign orchils. Many of them 

 have been known to the rural inhabitants from time 

 immemorial. The parti-coloured and often exceedingly 

 beautiful tartans of the Highland clans, used to be dyed 

 with the colouring matter derived from the common 

 grey foliaceous lichens which so plentifully clothe almost 

 every tree and wall-; and many an old woman in the 



