DYEING. 



217 



Colouring 

 Mat ten. 



Method of 

 correcting 

 the bad qua- 

 lities of wa- 

 ter. 



AliJies. 



Acids. 



Primary co- 

 lour*. 



the quantity of each. The salts of lime, the most com- 

 mon earth which is to be found in water, are easily de- 

 tected by the copious white precipitate which they af- 

 ford with the oxalate of ammonia, and those of iron by 

 a solution of galls. In general, water which contains 

 earthy or metallic salts, in such quantity as to be inju- 

 rious, readily decomposes soap by a double affinity : 

 the acid combines with the alkali of the soap, while 

 the earthy basis unites with the oil, and forming an 

 earthy soap which is insoluble, produces the curdling 

 appearance observable on such occasions. If then a 

 water be clear, destitute of taste and smell, and capa- 

 ble of dissolving soap readily, it may be regarded pro- 

 per for the objects of dyeing ; and all waters which 

 possess these properties are equally fit for the purpose. 

 But it may be remarked, that waters which hold the 

 earths mechanically suspended, that is, such as are 

 muddy, are less prejudicial than those which hold them 

 in solution : In the first case, they attach themselves 

 but loosely to the stuff; in the second, they are preci- 

 pitated in a state of minute division, and combining 

 with the mordant, become intimately united with the 

 cloth. 



49. But as it is not always in our power to choose 

 water of the best quality, means have been devised for 

 correcting the injurious properties of this fluid, when 

 it happens to be bad. The most common corrective 

 for this purpose is bran, which is allowed to remain in 

 the water until it acquires a sour taste. Twenty-four 

 bushels of bran are put into a tub or vat that will con- 

 tain about 10 hogsheads ; a large boiler is filled with 

 water, which, when just ready to boil, is poured into 

 the vat : the acid fermentation soon commences, and 

 in 21 hours the liquor is ready for use. Berthollet 

 conceives that the sour water acts by decomposing the 

 carbonate of lime and magnesia, from which its acid, 

 being more powerful, disengages the carbonic acid; and 

 that in this way the earthy sediment, which is occasion- 

 ed by boiling, is prevented from taking place. 



50. Alkali, the next solvent in order which we men- 

 tioned, is employed to dissolve several colours. In ge- 

 neral, as Chaptal remarks, the colours which are pro- 

 duced by fermentation are less soluble in water than 

 alkali. This is the case with indigo, woad, &c. Thus, 

 boiling water dissolves only a ninth-part of its weight 

 of indigo ; woad communicates very little colour to this 

 fluid ; and annotta can scarcely be dissolved in it at all 

 without the aid of alkali. After the colouring matter 

 has been extracted by an alkali, it is precipitated by 

 the addition of some weak acid. 



51. Acids are sometimes employed as solvents of co- 

 louring principles. The chemical blue of Farmer, or 

 the Siunit blue nf dt/ert, is a solution of indigo in con- 

 centr.ited sulphuric acid, (nihliche has proposed to 

 extract the colour of yellow wood, broom, turmeric, &c. 

 by means of the aceto-citric, and simple acetic acid, or 

 any other of the vegetable acids. The nitric and oxy- 

 muriatic acids impart a yellow tinge to all animal sub- 

 stances ; and the former of these acids is even su< 

 fully employed to communicate to silk and wools very 

 beautiful yellow colour. The nitric acid produces a si- 

 milar effect on madder ; but the yellow thus developed 

 disappears when the acid is neutralized. 



5'2. The simple colours, or at lea-st those obtained 

 from the decomposition of light by means of the pri-.ni, 

 are seven in number, viz. red, orange, yellow, green, 

 blue, indigo, and violet. These colours are supposed 

 to he homogeneous or simple, because when they are 

 transmitted a second time tlirough the prism, they are 



VOL. viu. PART i. 



refracted without undergoing any farther change. This Colouring 

 argument, however, is by no means conclusive; for if *J v 

 any coloured ray be composed of two others, each of *""V"" 

 these must have the same refrangibility, as it is in con- 

 sequence of this circumstance alone that they could 

 occupy the same place in the prismatic spectrum ; and, 

 in that case, a second refraction could not separate 

 them. The truth is, that the prism furnishes only 

 one means of the analysis of light, and merely shew* 

 that certain rays differ from others in refrangibility. 



53. Before the discoveries of Newton, the red, yel- 

 low, and blue, were generally supposed to be primary 

 rays, and the rest to be compound colours arising from 

 their intermixture. Thus, red with yellow, produces 

 orange ; yellow with blue, green ; and blue with red, 

 indigo. Another hypothesis has been lately advanced Hypothesis 

 by Prieur, and supported by very ingenious reasoning, of Frieur 

 According to him, the red, green, and violet, are the concerning 

 primary rays ; the red and green giving rise, by their colour - 

 intermixture, to orange or yellow, according as the for- 

 mer or the latter predominates ; the green and violet 

 forming blue'; and the violet and red purple ; and 

 thus, by various modifications of the three original co- 

 lours, all the others are obtained. We shall give an ex- 

 ample of the kind of proof which has been adduced in 

 support of this hypothesis. Let one of the rays, as 

 orange, be selected a colour which is supposed to be a 

 compound of red and green, the former being in ex- 

 cess. If this colour be really a compound of red and 

 green, and if the arrival of the rays to the part of the 

 spectrum which the orange usually occupies be pre- 

 vented, by the interposition of a substance that allows 

 only the red or the green rays to pass, we ought to 

 find beyond this substance only red or green ; on the 

 other hand, if the orange be simple, these rays will 

 pass through neither of the interposed substances, and 

 beyond this we should have only black. The results 

 corresponded with the hypothesis in a great variety of 

 trials. An additional argument was also derived for its 

 support, by combining the red, green, and blue rays, 

 and thus obtaining white light. 



54-. In a practical point of view, such opinions are Substnmive 

 of very little moment ; and perhaps it may be more nd adjei- 

 useful to attend to those distinctive characters of the ?' ve colour - 

 colouring principles, which are founded on their che- mg m 

 mical relations with the stuffs. Accordingly, Dr Ban- 

 croft has suggested the division of colouring matters 

 into substantive and adjective ; " the first including those 

 matters which, when put into a state of solution, may 

 be fixed with all the permanency of which they are 

 susceptible, and made fully to exhibit their colours in 

 or upon the dyed substance, without the interposi- 

 tion of any earthy or metallic basis ; and the second 

 comprehending all those matters which are incapable 

 of being so fixed, and made to display their proper 

 colours without the mediation of some such basis." 

 (I'/iiluxtinhy of Permanent Colours, vol. i. p. 1 18.) These 

 terms, though perhaps not altogether unexceptionable, 

 must at least be admitted to have a reference to real 

 differences between the substances which they are in- 

 tended to distinguish ; and though the advantages 

 which are derived from this arrangement are indeed 

 but slight, since very few colouring matters are con- 

 tained under the first head, but most of them under the 

 second, there can be no impropriety in adopting them, 

 particularly as they are intended to express a fact, and 

 not a theoretical opinion. In treating of colouring 

 matters, we shall therefore observe Dr Bancroft's distri- 

 bution of them ; but in describing the processes by 

 2 E 



