LITMUS.

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and lilac. The material known as litmus is also a produdt from lichens, andis obtained by a modified process of setting in adtion the colourable principlescontained in them. Litmus is rarely if ever used in dyeing, although itcolours silk blue and violet, but the colours produced are so fugitive as to beonly employed in extreme fancy styles.

Litmus was chiefly used for washing blues, for colouring wine, vinegar, andcordials, and is still employed by chemists for the preparation of the so-calledlitmus paper. The ordinary litmus of commerce is made from the Rocccllalecanora and variolaria, lichens obtained on the coasts of the Mediterranean,in Sweden, Norway, and the Canary Islands. The raw material is chieflysent to Holland for conversion into litmus, which is done by adding to thelichens, previously ground to a pulpy mass, a mixture of ammonia and car-bonate of potassa (pearl-ash). The paste is left to ferment until a violetcolour appears. When this stage is reached lime is added, and also anotherquantity of pearl-ash and some stale urine, when the mixture is left to fer-ment for some three weeks longer, until a blue colour is obtained. The pulpy,semi-fluid mass is rendered more solid by the addition of chalk and someplaster-of-paris, the excess ofliquor is separated by means of sieves, and afterhaving become sufficiently dry is shaped in jujube or lozenge form, driedeither in the open air (in summer) or in heated drying rooms, and deliveredto the trade in seven different qualities, No. 1 containing most, and No. 7 leastchalk. Litmus so prepared is a dry, light blue substance, the better thequality the less insoluble matter it contains.

The orchil prepared by the adtion of ammonia upon lichens does not become,when treated with acids, perfectly red, and that colour is not rendered blue bycontadt with ammonia and alkalies; the presence of carbonate of potassa asused in the manufadture of litmus has therefore the effedt of modifying theprocess of oxidation of the colourable matters, so as to result in the formationof a red acid, which, with alkalies, forms a blue salt. The blue colour of thelitmus of commerce is therefore due to the colour of a potassa salt of a redacid. The real colour of that acid is made apparent by the adtion of strongeracids upon litmus, the aqueous or alcoholic solutions of which all containan excess of alkali, which has to be saturated first before the colour beginsto turn to red.

Since litmus is used as a reagent of some value in chemistry its pre-paration so as to be most sensitive and delicate for that purpose may notbe here out of place: to the litmus solution is added a slight excess ofacetic acid; it is next carefully neutralised with ammonia, and then boiled fora long time so as to expel the last traces of any free volatile alkali. Allchemists are aware that litmus tindtures when kept in stoppered bottles be-come gradually discoloured in consequence of a peculiar redudtive fermenta-tion, but exposure to the oxygen of the air is sufficient to restore the originalcolour in full strength. Something similar takes place with orchil; both thesesubstances are similarly affedted by the adtion of reducing agents.

Kane and Gelis have studied the peculiar blue produced from the lichens aspresent in litmus. The former chemist has found in litmus from three to fourdistindt colouring matters, which he isolates by the following process:—Litmusis ground to a coarse powder and exhausted with boiling water; the insoluble,pale blue residue is treated with a slight excess of hydrochloric acid, whereby