a Yellow Colouring Matter. 193 



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 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 needles, sometimes 

 a quarter of an inch in length. The specimens 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 compared with the plants from other lo- 

 caUties. By this precaution, the alcohol 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 al- 

 cohol. The quantity of the lichen at my disposal has not 

 hitherto been sufficient 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 ob- 

 taining the colouring matter, or Parietin, 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 filter- 

 ing and adding fresh alcohol until the colour appeared to be 

 extracted. The solution has scarcely passed through the fil- 

 ter, before it begins to deposit the shiny scales of parietin. 

 If we attempt to purify these by redissolving them in alcohol, 

 we shall find that only a portion is dissolved, and the deposit 

 from the alcoholic solution, instead of presenting the lustre of 



VOL. XXXVII. NO. LXXII. JULY 1844. J( 



