H^ Dr. R. D. Thomson on Parietin and oji the 



these plants are supplied. Trees and lichens have no such 

 atmosphere, rich in salt, from which they can derive their 

 nourishment. They must be indebted for the inorganic food 

 which they contain to the soil upon which they grow; thence, 

 since lichens do certainly contain inorganic matter of various 

 kinds, as appears from the facts detailed in this paper, the in- 

 evitable conclusion is forced upon us, that these species of 

 plants are not only nourished by the atmosphere to which bo- 

 tanists have hitherto appeared to restrict their sources of food, 

 but that they are also capable of extracting inorganic matter 

 from the rocks and trees over whose surfaces this class of 

 plants is so extensively distributed. 



Preparation of Parietin. 



When the yellow Parmelia is digested in cold alcohol, of 

 spec. grav. 'S^O, a yellow fluid is produced, obviously from 

 the solution of the yellow colouring matter of the lichen. 

 When gently boiled the liquid becomes deeper coloured, and 

 when a sufficient quantity of alcohol is employed and the 

 liquor is allowed to evaporate spontaneously, the colouring 

 matter is deposited on the sides of the vessel in the form of 

 fine yellow needles, sometimes a quarter of an inch in length. 

 The specimens of lichen from which crystals of this descrip- 

 tion were extracted, were derived from a dry wall, and at a di- 

 stance from the neighbourhood of the sea. 



In order to procure the colouring matter of the yellow Par- 

 melia, it is proper to dry the lichen at a moderate tempera- 

 ture. This observation is particularly applicable to sea spe- 

 cimens, which are much more succulent than those derived 

 from inland localities. By this precaution the alcohol will 

 more completely extract the colouring matter, without violent 

 or long-continued boiling. We should probably succeed in 

 obtaining the purest product by removing as much as possible 

 of the water from the lichen, by drying in a stove and then 

 digesting in cold alcohol. The quantity of lichen, however, 

 at my disposal has not hitherto been sufficient to enable me to 

 accomplish the extraction in this manner. It has been stated 

 that the colouring matter may be obtained in the form of 

 needles, but generally it falls in the shape of brilliant yellow 

 scales as the alcoholic solution cools. 'I'he mode in which it 

 may be readily procured is, to boil the lichen gently with alcohol 

 for a few minutes, and then to filter and add fresh alcohol until 

 the colouring matter appears to be entirely taken up. The 

 solution has scarcely passed through the filter before it begins 

 to deposit the shiny scales o{ Parietin. If we attempt to pu- 

 rify these by redissolving them in alcohol, we shall find that 



