LITMUS.
383
Croton tinctorium pervades the entire plant at every time of its life, but ischiefly present in the cellular tissue, wherein it occurs as a colourable body.By the action of the oxygen of the air and rapid drying a blue colouration isproduced. When the fruit of this plant is immersed in twice its bulk of waterand heated to from 50° to 6o°, that liquid assumes a rather deep violet-bluecolouration, and deposits, on being evaporated, a beautiful azure-blue resinoussubstance. Acids turn the colour of the aqueous solution to a yellowish red,which is not rendered blue again by alkalies, but becomes greenish. By thisreaction, therefore, the “ litmus on rags ” is distinguished from the litmus ofcommerce. The researches of Dr. Langdale and Dr. Martius made with thejuice of the plant just described have proved that it dyes, without the aid ofmordants, a violet-red upon wool, silk, and cotton tissues, and that this colourmay be rendered fast by steaming and the simultaneous adtion of ammoniavapours, which, however, turn the colour more blue. The red colour of theouter crusts of some kinds of Dutch cheese is due to the presence of someladtic and butyric acids in that substance. No good substitute for this11 litmus on rags ” for the last-named purpose has as yet ever been found. Asum of ;£ro,ooo is annually paid by Dutch farmers, chiefly to the inhabitantsof Grand-Gallargues, for a commodity which, at first sight, no one wouldtake to be anything else but dirty rags, best suited for paper-making afterhaving been bleached. A portion of the rags, after having being used to rubcheese with, are sent back, because it has been found that the old rags takeup and develope the colourable matter far more readily than new ones. Theviolet-red colour prepared with the various kinds of lichens above mentionedis not the only one which these plants are capable of yielding.
M. Westring has made a series of dyeing experiments* chiefly with lichensgrowing in Sweden, a country wherein these cryptogamic plants are found ingreat abundance. Among the varieties he experimented with are :—ThePcrtusus ; sanguinarius, cincreus, rugosus, hcemaioma, subcorncus, tartarcus,Lichen auritus, centrofugus , saxatilis, parietinus, omphalodcs, saxicola, andmany others, all found on rocks or the barks of trees. In order to dye withthese lichens, M. Westring employs ammonia as the most active solvent, or amixture of lime (10 parts), with of sal-ammoniac upon 1 part of lichen.The addition thereto of a small quantity of common salt is sometimes useful,especially for silk, which thus acquires more lustre. Mordants were not used,as being found to do more harm than good. The colours obtained varied ac-cording to the mode of extraction applied, also according to the temperatureand the quality of the water employed.
The colouring matter of some of the lichens can be extracted by steepingthem in cold water, and the liquid obtained is immediately suited for the dye-beck; maceration in tepid water extracts the colouring matter from almostall the lichens. The author obtained the following shades upon divers tissues,by more or less varying the treatment above described:—Grey, nut-brown,more or less deep cinnamon, feuille morte, brick-brown, straw-yellow, olive,'brown, and violet. Some lichens, among them certain species of Usnea, walllichen, yield to solvents a yellow principle, about which more will be saidwhen the yellow colouring matters are treated.
* “ Annales de Chimie,” vol. xii., p. 240.