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 localities. 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 alcohol. 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 obtaining 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 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 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.

N

the substance as at first obtained, assumes the aspect of a brownish yellow powder.

COMPOSITION OF PARIETIN.

The product of the second solution in alcohol, when dried at 212°, and burned with oxide of copper, afforded the following result:

3.16 grains gave 7.376 carbonic acid.

As it appeared from the preceding result that the parietin was altered in its character, by attempting to redissolve it in alcohol; the parietin, after being dissolved in alcohol from the lichen, was, after the filtration of the fluid, allowed to deposit by cooling. It was then thrown on a filter, and dried on a tile, and then digested in hot alcohol, to remove any fatty or resinous matter with which it might be contaminated. The same object may be attained by digestion in ether. The parietin was then dried at 212°, and analysed.

2.96 grains afforded, when burned with black oxide of copper, 7.15 grains carbonic acid.

The formula, therefore, will be, according to this view,

or we may, as in the preceding case, consider it as an oxide of an oil, and the composition, when calculated, would be—

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 we then consider parietin as a resin, deriving its origin from an oil of the turpentine type, the preceding analyses may be classed as follows:

:

The effect of reagents upon parietin is striking. A very minute portion of the substance will impart its yellow colour to a large quantity of alcohol, and this solution is sensibly acted on by reagents. 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 becomes 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, sug

gests 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 the process may be reversed, by placing a few drops of the alcoholic solution 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, sufficiently intense 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 into test papers, and found that, when properly impregnated with the solution, they were little, if at all inferior to turmeric paper, in their delicate detection of ammonia. Test paper may be prepared extemporaneously from the alcoholic solution, when it is wished to detect ammonia, by dipping a piece of paper into the alcoholic solution, and then applying it in its wet state to the ammoniacal vapour. The yellow colour is immediately transformed into a reddish purple, but more distinct than the colour that becomes apparent in turmeric paper of old preparation, under similar circumstances, which is a dirty brown. One of the principal recommendations of the liquid test already noticed, is the circumstance of its being capable of preservation without undergoing deterioration, while the test papers which have been frequently recommended although possessing most delicate testing powers when freshly prepared, gradually lose their value by preservation. I believe this to be the explanation of the failure in this country of some continental test papers, which have been recently recommended. It would therefore appear, that the best test paper being that which is of fresh preparation, the most convenient source for its production is that from which it can be most rapidly procured in an efficient state. The observations which have been made upon parietin, in reference to its colouring powers, tend to show that it may be employed with advantage for the most delicate purposes to which turmeric is applied. Parietin, however, is not acted on by acids; the natural yellow colour merely

becomes brighter, while turmeric, which contains a blue and yellowing colouring principle, has the former reddened by acids, and the latter converted to a brown by alkalies. Moistened yellow parietin paper, on the other hand, becomes red or purple when freshly prepared, and reddish brown, if long prepared, by coming in contact with ammonia and other alkalies. The other reactions of parietin are simple. The alcoholic solution is precipitated yellow by nitrate of silver and acetate of lead, and other metallic salts. A solution of permuriate of iron renders the colour much darker. The precipitates with silver and lead have not been analysed, from the minute quantity of parietin at my disposal.

The yellow colour of the Parmelia parietina early attracted the attention of those persons interested in dyes. It was accurately described by Hoffmann, Amoreux, and Willemet, in 1786. The latter informs us, that the Swedes in the province of Oeland, obtained by means of this lichen and alum a yellow dye for woollen stuffs, and that a flesh tint was also procured from it, fitted for linen and paper; that goats eat this lichen; and that Haller recommended it as a powerful tonic in diarrhoea. He adds, that he had himself used it in his practice as a tisan, and had found it to prove beneficial in that form of the disease which occurs in autumn. Hoffmann states, that in Norway, when boiled with milk, it is used as a remedy in jaundice. This idea may have perhaps originated from the correspondence in colour of the disease and cure, upon the principle so much in vogue at present, "similia similibus curantur." Hoffmann affirms that he never could obtain a yellow colour from this lichen, but that with wine vinegar he obtained an olive-green or fawn colour; and with true wine vinegar (aceto vini vero) and copperas, a flesh or apricot shade. Of these colours he has appended to his essay specimens, together with forty-nine others, obtained from various species of lichens. Dr John P. Westring of Nordkoping, in Sweden, who prosecuted an extensive inquiry into the colouring matter of lichens, describes the Lichen parietinus

* Memoires Couronnés en l'anneé 1786, par l'Academie des Sciences, Belles Lettres, et Arts, de Lyon, sur l'Utilité des Lichens, dans la Medicine et dans les Arts. 8vo, 1787.