180 DYEING. 



in substances which they have less affinity for, than 

 when silk or wool are to be dyed. Thus iron dis- 

 solved in the sulphuric acid may dye wool ; but 

 when it is intended to dye cotton and linen by 

 iron, the latter should be dissolved in the acetous 

 acid. 



There are few colouring substances that have, 

 of themselves, so strong an affinity for cloth as to 

 answer the purpose of dyeing so as to remain per- 

 manent ; and, on this account, an intermediate 

 substance is employed, that has a decided attraction 

 for both the colouring matter and the cloth, thus 

 serving as a bond of union between them. This 

 substance is previously combined with the cloth, 

 which is then dipped into the solution containing 

 the dye-stuff. The dye-stuff combines with the in- 

 termediate substance, which being firmly combined 

 with the cloth, secures the permanence of the dye. 

 Substances employed for this purpose are deno- 

 minated mordants. Instead of this some prefer the 

 term basis. 



The most important part of dyeing consists in 

 the proper choice, and the proper application of 

 mordants; as upon them, the permanency of every 

 dye depends. What has been said respecting the 

 application of colouring matters applies equally to 

 the application of mordants. They must be pre- 

 viously dissolved in some liquid, which has a 

 weaker affinity to them than the cloth has, to 

 which they are to be applied \ and the cloth must 

 be dipped, or even steeped in this solution, in 

 order to saturate itself with the mordant. The 

 mordants are earths, metallic oxides, tan, and oil. 



Of the earths, alumine is the most useful. It is 

 applied in the state of sulphate of alumine, or 

 common alum ; and in that of acetate of alumine. 



