Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 1, 1802).djvu/304

-74J BL A cil : the former is collected for making varnishes, and the latter is used for lamps and torches. Ge- nuine naphtha is sometimes recom- mended in diseases of the nerves, but it is seldom obtained in a pure state. The solid bitumens are, amber, jet, asphaltum, or bitumen of Ju- dea, and fossil or pit-ccal. By dis- tillation, they all yield an odorous water, more or less coloured and saline ; an acid frequently in a concrete state, an oil similar to the native rock-oils, but which soon increases in weight, and becomes thicker ; and, lastly, a quantity of volatile alkali. The residuum is a charry matter, differing in appear- ance, according to the nature of the analyzed bitumen. Barbadoes tar is a bitumen of a consistence between a fluid and solid ; and turf or peat is, by some writers, supposed to belong to this class. It is conie&ured by naturalists, that all bitumens are of animal or vegetable origin ; and that the cir- cumstances by which they differ from the resinous and other oily matters of vegetables and animals, are the natural effects of time ; or of an alteration produced on them by mineral acids ; or of both causes combined. This opinion is the more probable, as bitumens, on a chemical analysis, afford oil and volatile alkali, neither of which is found in any other minerals. BLACK, the darkest ot colours, supposed to be owing to the ab- sence of light, as most of the rays which fall on black substances are not reflected, but absorbed by them. There a*e many shades or varie- ties of this colour. The native Mack iubstanees are, biack chalk, BL A pitcoal, black sands, black vege- table juices, and cuttle-fish ink. Those which are the product of fire, comprehend charcoal blacks, soot blacks, and black metallic calces. Blacks obtained by mixture, are those from iron, silver, and from a combination of lead with sulphur. The infusions of certain vegetable astringents, mixed with green vi- triol (which is a solution of iron m the sulphuric acid), produce a deep black colour, of most extensive use for dyeing and staining. The astringent substances chiefly em- ployed for this purpose, are the excrescences of the oak-tree, call- ed galls ; all parts of this tree, as the leaves, acorns, and more par- ticularly the bark and wood. A great variety of other vegetable substances, such as the small branches and flowers of the su- mach-tree, alder bark, bistort root, and, in general, these which are astringent or corrugating to the taste, possess simi.ar properties. The power by which these vegeta- bles strike black with vitriol, and their astringency, are proportional to one another, and seem to de- pend on one and the same princi- ple. Of the other properties of this astringent and colouring mat- ter, little more is known, than that it is dissolved and extracted both by water and spirit of wine, and that it does not exhale on the evaporation of the menstruum.—- See the article Dyeing. The only native vegetable black, is the juice of the cashew nut-tree, or Anacardium occidentale, which probably is the tree that yields the black varnish of China and Japan. -^-See Varnish. Lastly, there are also several colours artificially prepared for the use