184 DYEING. 



astringent principle, and, lastly, with the aluminous 

 mordant. This is employed in dyeing the Adria- 

 nople red. 



Several other substances are used as mordants 

 occasionally, either as principals, or to facilitate the 

 combinations of others with the cloths ; such as ni- 

 trate of bismuth, oxide of arsenic, corrosive sub- 

 limate, acetate of lead, sulphate, or acetate of 

 copper, &c. 



The chief use of mordants is to render the dyes 

 permanent, but they have also considerable in- 

 fluence on the colour produced : thus, the same 

 colouring matter will produce very different dyes, 

 according to the mordant used to fix it. If the 

 aluminous mordant be used for cochineal, the colour 

 will be crimson ; but if the oxide of iron be used for 

 the same colouring matter, black will be the result. 



It is necessary, therefore, to choose such mor- 

 dants and colouring matters as together shall pro- 

 duce the desired colour. And this principle 

 enables us also to produce various colours with 

 the same dye-stuff, only by changing the mordant. 



It is probable that the whole of the surface of 

 the fibres of cloth are not covered by the colouring 

 matters precipitated upon them ; but that the 

 particles of colour are at some distance from each 

 other. For cloth may be dyed different shades of 

 the same colour; that is, it may be dyed deeper a 

 second time than at first, by increasing the quan- 

 tity of colouring matter, which could not be the 

 case if the whole surface were covered. Another 

 circumstance renders this opinion probable ; all 

 those colours which dyers call compound are made 

 by dyeing the cloth first one colour, and then 

 another : thus, green is got by dyeing cloth first 

 blue and then yellow. 



