Dr. R. D. Thomson on Parktin, a Yellow Colouring Matter. 187 



which they can derive their food. They must be indebted for the 

 inorganic matter which they contain to the soil upon which they grow. 

 Hence, since lichens do certainly contain inorganic matter of various 

 kinds, as appears by the facts detailed in this paper, the inevitable 

 conclusion is forced upon us, that these species of plants are not only 

 nourished by the atmosphere, to which botanists have hitherto appeared 

 to restrict their sources of food, but that they are also capable of ex- 

 tracting inorganic matter from the rocks and trees over whose surfaces 

 they are so largely distributed as humble tenants. 



PREPARATION OF PARIETIN. 



When the yellow Parmelia is digested in cold alcohol, of -840, a 

 yellow liquid is obtained, obviously from the solution of the yellow 

 colouring matter of the lichen. When boiled gently the liquid be- 

 comes 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 needles, sometimes a quarter of an inch in length. The speci- 

 mens of lichen from which the best crystals of this description were 

 obtained, were from the neighbourhood of Glasgow, and were rather 

 dry, as if they had grown upon a dry wall, little exposed to moisture. 



In order to procure the colouring matter of the P. parietina, it is 

 proper to dry the plant at a moderate temperature. This is particularly 

 to be attended to with the sea specimens, which are succulent when com- 

 pared with the plants from other localities. By this precaution, the alco- 

 hol will more effectually 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 the lichen at my disposal has not hitherto been suffi- 

 cient to enable me to attempt to extract the colouring matter in this 

 manner, but I intend to do so on the first opportunity. I have stated 

 that I have succeeded in obtaining the colouring matter, or Farietint 

 as I propose to term it, in the form of needles, but generally it falls 

 in the shape of brilliant yellow scales, as the alcoholic solution cools. 

 The mode in which I have extracted it was by gently boiling for a 

 few minutes the lichen in contact with the alcohol, then filtering and 

 adding fresh alcohol until the colour appeared to be extracted. The 

 solution has scarcely passed through the filter, before it begins to 

 deposit the shiny scales of parietin. If we attempt to purify these 

 by re-dissolving them in alcohol, we shall find that only a portion is 

 dissolved, and the deposit from the alcoholic solution, instead of pre- 

 senting the lustre of the substance as at first obtained, assumes the 

 aspect of a brownish yellow powder. 



