155. Common sumach, (rims coriaria,) a shrub, grow ing naturally in Syria, Palestine, Spain, Portugal, and some parts of North America, yields a pale yellow dye, with the aluminous basis. The infusion, which is ob tained from the wood previously reduced to powder by a mill, is of a greenish fawn colour, but it soon becomes brown by exposure to the air.
156. A solution of potash produces vet v little change on it, while recent ; acids brighten its colour, and turn it yellow. A solution of alum reticle' s it turbid, and pro duces a small yellow precipitate. Of all astringents, sumach bears the greatest resemblance to galls. The precipitate which it produces with solutions of iron is less in quantity than what is obtained by an equal weight of galls, hut it is very copious, and may be substituted for them by increasing the quantity.
157 Sumach alone gives a fawn colour inclining to green ; but cotton stuffs, which have been impregnated with the acetate of alumine, take from it a very good durable yellow. It is principally employed for drab and dove colours in calico printing, and for dyeing black with iron, and the solutions of that metal.
158. The berries of the rhamnus infectorius give a lively yellow, but one which is destitute of durability. They are chiefly used for topical dyeing, in calico print ing. The colour which they communicate is so very fugitive, that the practice of dyeing from them should be abandoned.
159. Saw-wort, serratula tinctoria, affords a better dye, and may he used as a substitute for weld with the alurninous basis. It yields a bright lemon yellow of con siderable permanency, which may be heightened with the nitromuriate of tin and tartar.
Of the Durability of Colours.
160. In the view which we have hitherto taken of co louring matters, we have considered the mechanical methods by which they are separated from the substan ces with which they exist in a state of combination, and the general appearances which they exhibit with a va riety of chemical agents. In describing these appear ances, we treated them rather as modifications produced upon the original colouring matters, than as actual chan ges effected in their composition. It must be evident, however, that, from the nature of the chemical agents to which, in some instances, the colouring matters were submitted, decompositions must have taken place, and new compounds been formed ; and that these compounds must be more or less durable in proportion as they are capable of resisting the action of the substances to which they are afterwards to be most frequently exposed, viz. light, air, water, acids, alkalies, and soap.
161. The influence of light upon colouring matters has been long known; but it was reserved for the deli cate precision of modern analysis to discover the mode of its operation. To do this, a knowledge of the com position of light itself, the most subtile of all material substances, was no less necessary than an acquaintance with the constitution of coloured bodies. 'Without en tering into any detail on this subject, it will be sufficient to state the results which have been obtained by experi ment, 162. Light appears to consist of three kind of rays, ;which form one homogeneous compound in the solar beam. These have been termed calorific, colorific, and deoxidizing rays : (See CnEmisTity, vol. v.) It is the nature of the latter which we propose to consider more particularly at present. The deoxidizing rays exert their agency chiefly in occasioning decomposition. The element which they most frequently detach from a state of combination with other bodies, is oxygen; and from this property, indeed, they derive their name. Colour less nitric acid exposed to the rays of the sun soon be comes red and fuming, a state which is known to pro ceed from a partial separation of its oxygen. Oxymu riatic* acid gas is also decomposed by it, and its oxygen is set at liberty. Several of the oxides of the metals suffer like changes by exposure to light ; the oxides of gold and silver are in this way partially revived and re stored to the metallic state. Paper moistened with a so lution of the nitrate of silver is blackened in a few mi nutes, and the muriate of the same metal undergoes that change still more speedily.
163. The effects of light upon the colouring matter of vegetables are equally striking, and more immediately connected with the object under consideration. These r fleets ate also to be ascribed to the disengagement of o.) gem In the procesa of bleaching, the colouring matter of the thread is exposed to the agency of that pi in tple by the decomposition of the water which is ap plied to the stuff. The combination of oxygen with the col turing matter being a species of combustion, the co lour is g•adually destroyed, and the substance to be bleached at last rendered white. This theory receives a hal py illustration from the powerful agency exerted by the oxy muriatic acid in the destruction of colour. It appears, then, that light produces changes in colouring principles, rather by disengaging oxygen than by its own immediate operation. We shall therefore consider a little more particularly the action of that principle on colouring matter s.