60 Messrs. Stenhouse and Groves on Picrorocellin, [May 18^ 



axes of these rliomboidal plates correspond with that direction within 

 the film o£ iodine which has been spoken of as the axis of the cr3^stal : 

 and as iodine belongs to the trimetric system, this may be considered 

 the principal axis, as being the one in the direction of which the crystals 

 are prismatically developed to the greatest extent ; and it also appears 

 that when a ray of light passes normally through such a crystal it is 

 divided into two rays polarized respectively parallel with and per- 

 pendicular to the same axis ; and the one whose flame of polarization is 

 parallel with the principal axis suffers the least absorption. 



III. Picrorocellin.^'' By John Stenhouse^ LL.D.^ F.E.S._, and 

 Charles Edward Groves. Received April 27, 1876. 



Through the kindness of Mr. C. Lavers Smith, the eminent orchil 

 manufacturer of Spitalfields, we were furnished with a quantity of a lichen 

 which he had observed to have a very bitter taste, and which came into 

 the market through a Portuguese house. It is believed to ha^'e been 

 brouQ-ht from the West Coast of Africa : but our endeavours to ascertain 

 the exact locality have hitherto been unsuccessful. Prom the appearance 

 of the lichen it seems to grow on limestone rocks ; and Mr. W. Carruthers, 

 of the British Museum, and the Be v. J. T. Crombie, to whom we sub- 

 mitted it, pronounced it to be a variety of Mocella fuciformis, the ordinary 

 Mocella usually growing on trees. This lichen is remarkable for its in- 

 tensely bitter taste ; and the preliminary experiments showed that this 

 is due to the presence of a ci'ystalline compound which is but slightly 

 soluble in water. 



PicroroceUin. 



The lichen was accordingly first treated with water and hydrate of 

 lime, in the nsual manner, to extract the erythrin which it contains in 

 common with other varieties of Bocella, and the residue, after being dried 

 at the ordinary temperature, was extracted by boiling spirit. The alcoholic 

 solution, which contained the bitter substance together with chlorophyl 

 and various fatty and resinous impurities, was concentrated by distillation 

 until almost the w^hole of the alcohol was removed. When cold, the dark- 

 coloured pasty mass was pressed in a cloth, boiled up with a small 

 quantity of strong spirit, and allowed to cool, pressed, and again treated 

 in the same manner. By this means much of the chlorophyl and almost 

 the whole of the oily matters were dissolved out, leaving a dark green- 

 coloured crystalline product. In order to remove the last traces of chloro- 

 phyl from this, it was boiled up twice or thrice with benzine, in which 

 the crystals are only slightly soluble. 



The spent " weed " from this variety of lichen which has been exhausted 

 with ammonia in the ordinary process of the orchil manufacture also 

 yields the same crystalline substance, but it is much more difficult to purify 



