DYEING. 



The compounds of oxides and colour- 

 ing substances may be compared to many 

 other chemical compounds which are in- 

 soluble, when the principles of which 

 they are formed are properly proportion- 

 ed, but which are capable of being- su- 

 per-saturated by an excess of one of the 

 principles, and thence of becoming solu- 

 ble. Thus, a metallic oxide, united with 

 a colouring substance in excess, will pro- 

 duce a liquor, the colour of which will 

 be modified by the oxide ; whereas, 

 when the colouring matter is not in ex- 

 cess, the compound will be insoluble, or 

 nearly so : these effects are very evident 

 in the combination of iron with the as- 

 tringent principle. 



Some other saline substances, as well as 

 the metallic salts, are also employed as 

 mordants. The neutral salts, sal ammo- 

 niac, nitre, and particularly sea salt, act 

 as mordants, and modify colours, but it is 

 difficult to ascertain the manner in which 

 they act : salts with calcareous bases also 

 modify colours ; but as these modifica- 

 tions are nearly similar to those which 

 would be produced by the addition of a 

 small quantity of lime, it is probable they 

 are decomposed, and that a little of the 

 lime enters into combination with the co- 

 louring particles and the stuff. 



Astringent substances are often em- 

 ployed as mordants. Tan, or the astrin- 

 gent principle, having a strong affinity 

 for cloth, and for colouring substances, is 

 found very useful for this purpose. It is 

 commonly prepared by infusing nut-galls 

 in water ; the cloth is immersed in this 

 solution, and allowed to remain till it is 

 sufficiently impregnated with the tan. 

 Sumach, which consists of the shoots of 

 the rhus coriaria (a shrub that grows in 

 the southern parts of Europe,) is often 

 used and prepared in the same way as 

 nut-galls. 



Animal substances are sometimes used 

 as mordants; in the process for dyeing 

 the Turkey red, the cotton stuff is im- 

 pregnated with animal oil ; and it is pro- 

 bable linen and cotton would take other 

 colours better, after some similar prepa- 

 ration. 



Exsiccation favours the union of the 

 substances which have an attraction for 

 the stuff, and the decompositions which 

 may result from that union ; but the ex- 

 siccation should be slow, that the sub- 

 stances may not be separated before their 

 mutual attractions have produced their 

 effect. 



To judge, of the most advantageous man- 



ner of employing mordants, we must first 

 pay attention to the combinations which 

 may be produced, either by the action of 

 the substances which compose them, or 

 by that of the colouring matter and the 

 stuff: secondly, to the circumstances 

 which may concur in bringing about these 

 combinations more or less quickly, or 

 in rendering them more or less com- 

 plete ; thirdly, to the action that the 

 liquor in which the stuff is immersed 

 may have, either on .its colour or tex- 

 ture ; and in order to foresee what that 

 may be, it is necessary to know the pro- 

 portions of the principles which enter into 

 the composition of the mordant, and what 

 will be, left in an uncombined state ia 

 the liquor. 



Of the action of air and light on colours. 



The action of atmospheric air on co- 

 lours is chiefly owing to the oxygen it 

 contains ; this Berthollet has shewn in, 

 some cases to have similar effects to a 

 slight combustion ; as when the air ren- 

 ders a substance yellow, fawn-coloured, 

 or brown, which he supposes it does 

 by the oxygen combining with the hy- 

 drogen of the stuff, and leaving the 

 charcoal predominant, which then com- 

 municates its own colour to it. The 

 action of the air in bleaching he sup- 

 poses to be caused by the combination- 

 of its oxygen with the colouring matter 

 of the stuff, which renders it soluble 

 in alkaline lixivia ; which for this reason 

 should always be used alternately witlt 

 exposure to the air. 



The changes which occur in the colours 

 produced by the union of colouring mat- 

 ter with metallic oxides are effects com- 

 pounded of the change that takes place 

 in the colouring matter, and of that which 

 the metallic exide undergoes. 



The light of the sun considerably ac- 

 celerates the alteration of colours ; this, 

 according to Berthollet, it effects, by fa- 

 vouring the combination of oxygen, and 

 by the combustion thereby produced. 

 Mr. Sennebier (who has published many 

 interesting observations on the effect o 

 light on colours,) on the contrary, attri*. 

 butes these effects to a direct combina- 

 tion of light with the substances ; but 

 Berthollet established his opinion by a 

 number of accurate experiments, wnich 

 give it a decided superiority. 



Colouring substances resist the action 

 of the air more or less, according as they 

 are more or less disposed to unite with. 



