692

LITMUS-ASSAY OF LICHENS FOR COLOUR.

(1525) Litmus is obtained principally from the Roccella tinctoria: it is prepared by a process similar to that used for archil, but it is moistened with a mixture of carbonates of ammonium and potassium,-the addition of sodic or potassic carbonate being essential to the development of the blue colour. During its preparation the mass at first assumes a red colour, but it ultimately becomes of an intense blue. It is made up with chalk or plaster of Paris into small cakes for the market. A small portion only of the colouring matter of litmus is soluble in water and in alcohol, but it is nearly all soluble in alkaline liquids. V. Luynes has succeeded in obtaining the essential colouring matter of litmus in a pure form by acting upon orcin (1530); I part of orcin, 25 of crystallized sodic carbonate, 5 of water, and I part of a strong solution of ammonia, are digested in a flask for 4 or 5 days, at a temperature of from 140° to 170°, with frequent agitation. The blue liquid thus obtained is to be diluted with water, and hydrochloric acid added in slight excess. The colouring matter is thus precipitated, and after washing and drying furnishes pure litmus in irregular fragments, assuming a metallic lustre when pressed. It is scarcely soluble in cold water, benzol, or oil of turpentine, but readily soluble in alcohol, with which it furnishes a red solution, which, when neutralized with an alkali becomes blue, and reddens again with very slight traces of free acid.

(1526) Assay of Lichens for Colouring Matter.-The different lichens contain variable amounts of colour-producing ingredients; consequently their commercial value is liable to great variation. Stenhouse estimates the comparative value of different samples by means of a standard solution of chloride of lime. A given weight of the lichens in powder is digested with slaked lime and water in order to extract the colorific acid, and on adding chloride of lime to this liquid a red colour is produced, which disappears in a few minutes, leaving a brownish yellow tint; fresh quantities of the chloride are added so long as this red coloration is produced by it. Another method also suggested by the same chemist consists in adding an acid to the solution of the lichen acids in lime water, and collecting and weighing the precipitate of the lichen acids. The colour-producing acids may be extracted by the following method (Stenhouse) :-The pulverized lichens are mixed with water, and, after standing for an hour, a quantity of slaked lime is added, and the mixture is allowed to digest for some time; it is then placed in a linen filter supported in a tin vessel with a perforated bottom, and the liquid is gradually displaced by the cautious addition of water, the

COLOURING PRINCIPLES OF THE LICHENS.

693

washing being continued so long as the filtrate yields the characteristic purple-red colour with solution of chloride of lime. On the addition of hydrochloric acid to the filtered solution, the lichen acids are precipitated in the form of a white gelatinous deposit, from which the water may be removed by pressure. The acids may then be crystallized from warm alcohol, but they must not be boiled with alcohol, otherwise compound ethers would be formed, into the composition of which the acids enter. This remarkable power of etherifying alcohol is a striking chemical peculiarity of the lichen acids.

The formula which indicate the composition of the numerous derivatives of the lichens are at present variously represented by different chemists. They are founded chiefly upon the analyses of Schunck and of Stenhouse, and except the first four formulæ, those in the first column are proposed by Gerhardt :

(1527) Erythric Acid, or Erythrin (Є20H22010).—This compound appears to be present in most of the lichens which yield archil. Berthelot and V. Luynes regard erythrin as the diorsellic ether of the tetratomic alcohol erythrite (1243)

Erythrin.

:

10

Erythrin is soluble in about 240 parts of boiling water, but is nearly insoluble in cold water; hot alcohol also dissolves it, and deposits the acid in stellate groups of needles. It is likewise soluble in ether. When heated in a tube it is decomposed, and orcin is sublimed. The alkalies dissolve it with facility. When boiled

* See Schunck (Liebig's Annal., xli. 157; liv. 257; lxi. 64); Rochleder and Heldt (Ib. xlviii. 1); Stenhouse (Phil. Trans., 1848, p. 63; 1849, p. 393); V. Luynes (Ann. de Chimie, IV. ii. 385).

694

PICRO-ERYTHRIN-EVERNIC AND LECANORIC ACIDS.

with baryta, baric carbonate is formed, whilst erythrite and orcin are liberated :

If the decomposition be arrested before it is complete, the body termed picro-erythrin is obtained :

12 16

16 7

Picro-erythrin (EH) is a colourless substance which is sparingly soluble in cold water, but very soluble in boiling water, from which, on evaporation, it may be obtained in confused crystals. Picro-erythrin may be regarded as the monorsellic ether of erythrite (V. Luynes). It has a very bitter taste. Its solutions give a precipitate with basic acetate of lead, but none with the normal acetate with ferric chloride it produces a beautiful purple colour. If heated alone in a tube it yields a sublimate of orcin. Alkalies dissolve it readily without the aid of heat. Its ammoniacal solution quickly becomes red by exposure to the air. When boiled with milk of lime or with baryta water it is decomposed, calcic or baric carbonate is formed, and orcin and erythrite (1243) are produced :

(1528) Evernic Acid (HЄ17H15,?) occurs associated with usnic acid in the Evernia prunastri. It is scarcely soluble in water, even when at a temperature of 212°, but is readily soluble in alcohol and in ether; with potassium and barium it yields soluble crystallizable salts. When boiled with lime or baryta in excess, orcin is formed, and a new acid termed evernesic acid :

Evernic acid.

=

2

Evernesic acid.

10 4

Orcin.

8

Ca€Ꮎ, + € He, + €H Ꮎ . Evernesic acid does not yield orcin; ammonia does not produce any red colour with it; nor does chloride of lime yield with it a red or violet colour.

(1529) If the Lecanora tartarea and some other lichens be treated with ether, and the solution be evaporated, white crystals of lecanoric acid (E16H110,?) are deposited; this substance is nearly

14 7

*Lecanoric acid, according to Gerhardt, is identical with the alpha and beta orsellic acids of Stenhouse.

ORSELLESIC ACID-LICHEN COLOURS-ORCIN.

695

insoluble in cold water, but it is freely dissolved by hot alcohol and by ether; its solution reddens litmus. Ferric chloride strikes a dull purple red with its spirituous solution. If boiled with alcohol it produces a true compound ether. The caustic alkalies dissolve lecanoric acid, and deprive it of the elements of carbonic anhydride, leaving orcin in solution. Orcin is also obtained by simply heating lecanoric acid and distilling. If lecanoric acid be treated with ammonia and exposed to the air, the acid combines with the elements of ammonia, and a purple colour is gradually developed, owing to the production of orcein.

Alpha-orsellesic, or Orsellic Acid (HE,H,,), so named from the French orseille, archil, is one of the products of the decomposition of lecanoric acid when its solution is boiled for a short time with an excess of lime or of baryta; by prolonged boiling it becomes converted into orcin ::

5' 8 7

When either lecanoric or erythric acid is boiled with alcohol for some hours, orcin is separated, and a substance termed pseudo-erythrin, or lecanoric ether, more properly orsellesic ether (Є2H ̧‚¤ ̧н‚Ð ̧), is formed: it crystallizes in plates or needles, which may be purified by recrystallization from boiling water. It is scarcely dissolved by cold water, but is very soluble in alcohol and in ether, as well as in alkaline solutions, from which it is precipitated unaltered on the addition of an acid. Its ammoniacal solution, when exposed to the air, assumes a wine-red tint.

2 21 14

2

In certain cases Schunck found an acid which he terms parellic acid (H2H110,H,O?), accompanying lecanoric acid: it is a fusible compound, which does not yield orcin when treated with alkalies. It crystallizes in brilliant, colourless, heavy needles, which are very sparingly soluble in cold water, but freely soluble in ether and in alcohol. The alcoholic solution has a very bitter taste; water precipitates the acid from it in a gelatinous form. It forms a white insoluble barium salt.

8 2 2

(1530) Orcin (E,H ̧,,H2O; Gerhardt); Sp. gr. of vapour 5*7.* -This substance appears to exist ready formed in several lichens,

*This vapour density is anomalous (E,H,2)2, yielding 3 instead of either 2 or 4 volumes of vapour, according to the experiments of Dumas, which, however, it is desirable to repeat under variation of temperature and pressure.

696

2

ORCIN-ORCEIN.

but it is generally procured by decomposing evernic, erythric, or lecanoric acid, by boiling it with an alkali. Stenhouse prepares orcin by boiling the solution obtained by treating one of the varieties of the Roccella or Lecanora with lime for some hours, and concentrating it by evaporation. The excess of lime must then be removed by means of a current of carbonic anhydride, and the filtered liquid evaporated to the consistence of a syrup; this residue must be treated with boiling alcohol, and allowed to crystallize; the red crystals of orcin thus obtained must be purified by recrystallization from their solution in ether. Orcin is a substance with a sweet but mawkish taste. It crystallizes with H2 in colourless, quadrangular prisms, and is readily soluble in alcohol, ether, and water. When heated to 212° it melts and gives off its water of crystallization, and at 550° (288° C.) it may be sublimed unchanged. It is not susceptible of the vinous fermentation. In its anhydrous form it is metameric with saligenin. With ferric salts it gives a dark red precipitate; it gives no precipitate with normal acetate of lead, but with basic acetate of lead a white precipitate is occasioned (E,H,Pb,,Pb). With chloride of lime orcin gives a purple-red colour, which quickly changes to a deep yellow. Nitric acid converts orcin into oxalic acid, and a resinous substance. If orcin be heated with stearic acid to 482° (250° C.) the two bodies unite, forming a wax-like body, analogous to the compound of mannite with the fatty acids.

3

2

If orcin be acted upon by the oxychlorides of the acids, substitution-products, such as diacetyl orcin, €,H,(Є,H ̧),,, may be obtained. Orcin also forms unstable compounds with bases, and dissolves considerable quantities of baryta and lime.

Orcein (Є,H,NO3).—If orcin be exposed in a shallow dish under a bell-glass with air to the vapour of ammonia, it yields a fine red colour; oxygen is absorbed, and a compound is obtained which has received the name of orcein :

2 ¤‚H ̧Ð1⁄2 + 2 H ̧N + 3 →2 = 2 ¤,H,NO3 + 4 H2O.

8 2

2

The aqueous solution of the result of the foregoing reaction contains orceinate of ammonia, and must be mixed with acetic acid, when orcein falls in red flocculi. Orcein is sparingly soluble in water, from which it is completely precipitated on the addition of a neutral salt. Alcohol dissolves it freely, forming a scarlet solution. It is sparingly soluble in ether. Potash and ammonia dissolve it readily, forming a splendid purple colour, which is the basis of the ordinary archil of commerce; the addition of common