Dr. R. D. Thomson on Parietin, a Yelhw Colouring Matter. 189 



Atoms. 



Carbon, 40 65-21 



Hydrogen, 16 434 



Oxygen, 14 3045 



and the formula 



C40 H|8 On 



exhibiting a stage in the oxidation of an oil similar to what we meet 

 with in the gradual production of resins from oils of the turpentine 

 type. In some respects the colouring matter under discussion 

 resembles a resin, and especially in its appearance when precipitated 

 from its solution in alkalies by an acid. If wo then consider parietin 

 as a resin, deriving its origin from an oil of the turpentine type, the 

 preceding analyses may be classed as follows: — 



Oil of parietin, C40 H,6 



Parietin, C40 H,6 Ou 



Oxide of parietin C^o H,o 0,6 



The effect of re-agents upon parietin is striking. A very minute 

 portion of the substance will impart its yellow colour to a large quan- 

 tity of alcohol, and this solution is sensibly acted on by re-agents. 

 When to such a solution a drop or two of nitric, or muriatic or sulphuric 

 acids are added, the yellow colour imparted to it by the parietin be- 

 comes much heightened, and even a very small proportion (much more 

 minute than that mentioned) will effect a sensible change. When the 

 solution is strong, the addition of acid produces a yellow precipitate. 

 When caustic ammonia, in the smallest quantity, is dropped into, or 

 applied by means of a rod, to a solution of parietin, the yellow colour 

 immediately becomes a rich red, inclining to purple. The same result 

 is obtained with caustic potash, caustic barytes, carbonate of soda, 

 caustic lime, &c. 



PARIETIN AS A TEST OF ALKALIES. 



The extreme delicacy of parietin in detecting alkalies, suggests its 

 utility in the laboratory. An alcoholic solution may be kept for use, as 

 the addition of a drop or two of the solution to a considerable quantity of 

 an alkaline liquor, will be immediately followed by a change to red; or 

 tlie process may be reversed, by placing a few drops of the alcoholic solu- 

 tion in a test-glass, and adding to it a drop or two of the alkaline liquor. 

 The alcoholic solution may be prepared simply by digesting the lichen 

 in cold alcohol, of sp. gr. -840, as I have found that a small portion of 

 lichen will impart a colour to a large quantity of alcohol, suflSciently in- 

 tense to serve as a very delicate test for alkalies. Observing the strong 

 colour that the alcoholic solution imparted to the filtering paper 

 which was used to purify the solution when first prepared, I cut these 



