134

BLEACHING.

dots, as passing along its whole length. Fig. 116 is a section of the brickwork, with thetube, burner, and receiver, as will be described hereafter. The tube may be called thetar main, as it is intended to be filled with tar : it is constructed if cast iron, and from itissue several (in this figure twenty-four) jets or burners, c, c, c; any other number maybe employed, d is a furnace under the tar main, the flue of which extends along, forthe purpose of heating the tar to the boiling point, in order to facilitate the process.From the main, b, the tar flows into the jets c; wicks are introduced into the jets, and,when set fire to, by a red-hot stick, will burn and emit a very considerable quantity ofsmoke; which it is the object of this apparatus to conduct through-many passages, forthe purpose of collecting its sooty particles.

There are a number of hoods, e, e, e, or bonnets, as they are termed, all of which,through their-pipes, have communication with, or lead into a main chimney,/!/'. Intothese hoods or bonnets the smoke of the .burners ascends, and from thence passes intothe main chimney /j and thence through the smoke tubes into the box g: here theheaviest particles of the black deposite themselves; but, as the smoke passes on throughthe farther pipes, a deposite of the second, or finer, particles of black takes place in thebox h. From hence the smoke proceeds through other pipes into a series of canvasshags, i, i, i, which are proposed to be about eighteen feet long, and three ia diameter.These bags are connected together at top and bottom alternately, and through thewhole series the smoke passes up one bag and down the next, depositing fine black,called spirit black, upon the sides of the canvass. After the jets have continued burningfor several days, the bags are to be beaten with a stick, so that the black may fill to thebottom ; and, when a sufficient quantity has accumulated, the bags may he emptied andswept out. Thus seventy or eighty bags may be employed; so that the smoke shouldpass through a length of about 400 yards, the farthest of which will he found to containthe finest black. The last bag should he left open, in order to allow the vapor to escapeinto the open air.

The main tar tube will require to he emptied every four or five days, in order toclear it from the pitchy matter that may have subsided from the burners, and they alsowill require to be frequently poked with a wire, to clear off the black which forms uponthe edges, and to drive down the carbonized tar which attaches itself to the upper partof the jets.

BLEACHING ( Blanchimmt, Fr.; Bhichen, Germ.) is the process by which the tex<-tite filaments, cotton, flax, hemp, wool, silk, and the cloths made of them, as well as va-rious vegetable and animal substances, are deprived of their natural color, and renderednearly or altogether white. The term bleaching comes from the French verb Nmichir, towhiten. The word blanch ., which has the same origin, is applied to the whitening ofliving plants by making them grow in the dark, as when the stems of celery are coveredover with mould.

The operations which the bleacher has recourse to (filler according to the nature ofthe bleaching means, the property of the stuft'to be bleached, and local customs o% cir-cumstances; and the result is also obtained with more or less rapidity, certainty,economy, and perfection. The destruction of the coloring matters attached to the bodiesto be bleached is effected either by the action of the air and light, of chlorine, or sulphur-rons acid; which may be considered the three bleaching powers employed for manufac-turing purposes.

Bleaching by the influence of air and sunshine is the most ancient, and still the mostcommon, method in several civilized countries; it is also supposed by many to be theleast injurious to the texture of yarn and cloth. The operations it involves are verysimple, consisting in the exposure of the - goods upon a grass-plat to the sky, with theiroccasional aspersion with moisture if necessary, in addition to the rain and dew. Theatmospheric air effects the bleaching by means of its oxygenous constituent, which com-bines with the coloring matter, or its elements carbon and hydrogen, and either makes itnearly white, or converts it into a substance easily soluble in water and alkaline solutions.This natural process is too slow to suit the modern demands of the cotton and linenmanufacturers. Fortunately for them, a new bleaching agent, unknown to ourfbrefathei s,has been discovered in ehlorine, formerly called oxvmuriatic acid, an agent nullified bychemistry so as to give an astonishing degree of rapidity, economy, and perfection, to thisimportant art. It is, however, not a little surprising, that the seienee which has so great'lyadvanced its practical part should have left its theory far from complete, and should aftor®no satisfactory answers to the two following questions.—What is the action of the solarrays upon the coloring matter ? How do'air and chlorine operate upon this principle .Some suppose that light predisposes the coloring matter to’combine with oxygen; othersfancy that it acts merely ia the manner of a high temperature, so as to determinereaction between the elements of that substance, and to cause a new combinatmpossessed of peculiar properties. It is generally admitted at the present day >. tportion of the oxygen of the air passes into the coloring matter, and changes its «