DYEING. 



oxygen, and thereby to suffer more or 

 less quickly a smaller or greater degree 

 of combustion : light favours this effect ; 

 but the colouring matter, in its separate 

 state, is much more prone to this com- 

 bustion than when united to a substance, 

 such as alumen, which may cither de- 

 fend it by its own power of resisting 

 combustion, or, by attracting it strong- 

 ly, weaken its action on other substan- 

 ces, which is the chief effect of mor- 

 dants; and this compound acquires 

 greater durability when it is. capable of 

 combining intimately with the stuff. 

 Thus the colouring matter of cochineal 

 dissolves easily in water, and its colour is 

 quickly changed by the air; but when 

 united to the oxide of tin, it becomes 

 much brighter, and almost insoluble in 

 water, though it is still easily affected by 

 the air, and by oxygenated marine acid : 

 it resists the action of these. better, how- 

 ever, when it has formed a triple com- 

 pound with a woollen stuff. 



Oxygen may unite in a small propor- 

 tion with some colouring substances, with- 

 out weakening their colour, or changing 

 it to another : thus indigo, which becomes 

 green by uniting with an alkali, with 

 lime, or a metallic oxyde, resumes its 

 colour, and quits those substances, when 

 it recovers a small portion of the ox- 

 ygen which it had lost. The liquor of 

 the whelk, employed to dye purple, is 

 naturally yellowish ; but when exposed 

 to the air, and more especially in the 

 sunshine, it quickly passes through vari- 

 ous shades, and at length assumes that 

 colour so preoious in the eyes of the an- 

 cients. 



It may be considered as a general fact, 

 that colours become brighter by their 

 union with a small portion of oxygen ; for 

 this reason it is found necessary to air 

 stuffs, when they come out of the bath, 

 and sometimes even to take them out of 

 it from time to time, expressly for this 

 purpose ; but in some cases the quantity 

 of oxygen, which thus becoming fixed, 

 contributes to the brightness of the co- 

 lour, is very inconsiderable, and its dete- 

 rioration soon commences. 



The action of the air affects not only the 

 colouring matter and the stuffs, but also 

 metallic oxyde, when they are employed 

 as Lntermedes, because the oxides are 

 deprived at first by the colouring matter 

 of part of their oxygen, and absorb it 

 afterwards from the air. Those oxides, 

 then, whose colour varies in proportion to 

 their quantity of oxygen, cause changes 

 of colour in the stuff in this manner. 



Thus the blue given to wool, by sul- 

 phate of copper and logwood, soon 

 changes into a green by the action of the 

 air ; because the copper, which is blue, 

 when combined with a small portion of 

 oxygen, becomes green, by its union with 

 a larger quantity. 



Colouring matter, in a state of combi- 

 nation with most substances, is less liable 

 to be changed by the air than when un- 

 combined; but there are some excep- 

 tions ; for alkalies produce a contrary ef- 

 fect : they darken the colours to which 

 they are added, and are found by experi- 

 ment to promote the absorption ot air, 

 and in proportion as this takes place, the 

 colours in general become more and more 

 brown. This is consonant to the effect 

 they produce on other substances, such 

 as sulphur, for they favour the absorp- 

 tion of air, because they have a strong at- 

 traction for the substance which is the 

 result of that absorption. 



Of the differences between Wool, Silk, Cm- 

 ton, and Linen, and the operations bij 

 which they are prepared for Dyeing. 



Wool and silk are animal substances* 

 cotton and linen are vegetable produc- 

 tions. Animal substances have a greater 

 disposition to combine with other sub- 

 stances, than those of vegetable origin ; 

 hence tiey are more liable to be destroy- 

 ed by different agents, and are nore dis- 

 posed to combine with colouring parti- 

 cles. Berthollet accounts for these pro- 

 perties by their principles being more 

 disposed to assume a gaseous form, and 

 having less cohesive force amcng them- 

 selves. Thus the pure or caustic alkalies 

 destroy animal substances, because they 

 combine with them, and thereby lose 

 their causticity. For this cause animal 

 substances cannot bear leys, and alkalies 

 should be used with great caution in 

 the processes for dyeing them ; whereas 

 no danger is to be apprehended from the 

 use of alkalies with vegetable sub- 

 stances. Nitric and sulphuric acids have 

 also considerable action on animal sub- 

 stances. 



Silk appears to bear some resemblance 

 to vegetable substances, by being less 

 disposed to combine with colouring par- 

 ticles, and by resisting the action of alka- 

 lies and acids more powerfully; but 

 though the action of these substances on 

 silk is weaker than upon wool, they 

 should still be employed with great 

 caution, because the brightness of co- 

 lour in silk appears to depend upon tb* 



