4. The fourth, or acid process, is pretty much the same for cottons as for linens ; the quantity of sulphu ric acid should amount to about nth or Ath of the weight of the cotton goods. It must be diluted with water till its specific gravity be reduced to 1.010. A steep in this diluted acid from six to twelve hours after each of the two last oxymuriatic pro ceases is generally made use of.
When the fabrics are very light, or contain dyed colours, the souring is only once used, and the strength of the acid is reduced to 1.008. This sour ing is applied after the last oxymuriatic process. In this case thorough washing or rinsing in water, is most strictly to be attended to before exposure to the sulphuric acid in sowing. When cotton fabrics are intended for printing with madder colours, they 'require one or two additional bucks or boils, and the sours should be one-third stronger.
The intermediate washings in pure water, and the proper draining of the goods after washing, must be observed with as much care in bleaching cotton as in bleaching linen. It is only in the bleaching of those goods intended for printing, that any exposure to the light and sits is now used, and that but seldom even in this case.
The rags to be whitened should be well washed in the engine, and when reduced to what is called half. stuff, the water should be run off, leaving just enough to allow them to be easily turned. While the rags are thus preparing, a solution of: the bleaching-pow der is to be got ready, by putting the powder into a pitcher or other convenient vessel, and pouring up on it two or three gallons of water, stirring and bruising it well, till every thing soluble is taken up. After it has stood some time to allow the insoluble sediment to fall down, it is fit for use, and the pure solution should be poured into the engine. The se diment may be repeatedly washed with fresh por tions of water to exhaust any remains of soluble matter, which alone is useful in the whitening pro cess. While this last operation is going on, the engine is to be kept moving, and to continue so for about an hour, which will generally be sufficient to produce the requisite degree of whiteness. The water may now be returned upon the engine, and the washing continued as usual' till the process be completed. The quantity of powder usually allow ed, is from two pounds to four pounds for evens hun dred weight of rags, in proportion to the whiteness ch re uired, and the difficulty of whitening the stuff.
tags containing dyed colours, to be discharged, shoull be well washed and reduced to half-stuff. They are then removed from the engine and put into a puncheon, made water tight, but having a sufficient opening in the side to admit with ease the putting in and taking out of the stuff; and capable of being shut up so as to retain the water. Having put the stuff
into this puncheon, take for every cwt. of the rags a solution containing from five to eight pounds of bleaching powder, according to the strength and fixedness of the colours to be discharged. Pour the solution into the puncheon among the stuff; allowing liquid enough to let the stuff float easily, and for each pound of powder used, add half a pound of sul phuric acid. Then shut up and secure the opening so as to make the puncheon air tight; then turn the puncheon round upon its axis, by means of a crank fixed at one end of it. Moving it in this manner gives facility and uniformity to the discharging pro cess.
We have now finished the sketch which we pro posed to give of the processes at present followed by the practical bleachers of Great Britain. For several other applications of the same operations, we refer to the article BLEACHING in the Encyclopedia. But probably a few words will be expected from us on the theory of the art. Upon this subject, the fol lowing observations are all we have to offer : The fibres of hemp, linen, and cotton, are naturally white; but, before bleaching, they are combined with a substance which gives them their grey or brown colour. The object of bleaching is to remove this substance. From the experiments which have been made upon the subject, it would appear that this substance is partly in the state of resin, and partly in a state analogous to that of the volatile oils. Re sins dissolve readily in the alkalies, which they neutralize and convert into a species bf soap. But this is not the case with the colouring matter, which is in a state analogous to that of volatile oil. After the weaver's dressing has been removed, the cloth is boiled or bucked repeatedly in alkaline leys, which dissolve and separate the whole of the colour ing matter, which possesses the characters of resin. • The alkaline ley, after this process, is turbid, has a brownish red colour, a strong smell, and has lost its alkaline properties. When muriatic acid is poured into it, a copious flocky precipitate subsides, consist ing of the colouring matter. This substance, when ' properly edulcoPated, has a greenish grey colour, which it retains when separated from the water by the filter. But, when dry, it becomes blackish ex ternally, though it retains internally its greenish tint. It is soluble in alcohol, insoluble in oil of turpentine, but dissolves readily in the alkalies. It tinges the strong acids, but does not readily dissolve in them. When thrown upon a iron, it burns with a yellow flame and a black smoke, leaving a cherry re siduum. These properties, for the knowledge of which we are indebted to Mr Kirwan i• are sufficient to show us that the substance which the alkalies se parate from linen is analogous to the resins.