18 Dr. TJiomas Thomson on [Jan. 



way the indigo which is at first loosely laid upon the fibres, 

 and easily removeable by washing, is slowly combined with 

 them, and thus becomes fixed on the cloth. A large quan- 

 tity of iron is necessarily attached to the cloth during this 

 process, and a continued action of sulphuric acid is neces- 

 sary for its removal. 



A third process consists in dissolving powdered indigo in 

 a hot solution of potash, and stannite of potash, or by boil- 

 ing it in potash or soda, along with metallic tin. It is 

 then precipitated in a white state by muriatic acid, and 

 the precipitate being thickened and mixed with fresh 

 chloride of tin, is printed on the cloth. When dry, the 

 piece is immersed in a solution of carbonate of soda. The 

 indigo becomes yellow by combining with the soda, and in 

 this soluble condition, attaches itself permanently to the 

 cloth. It soon afterwards becomes blue, by the absorption 

 of oxygen from the atmosphere. 



11. Catechu Brown. — This im- 

 portant dye-stuff, formerly known 

 by the name of terra japonica, is 

 procured by boiling the brown heart 

 wood of the acacia catechu, or 

 khair-tree. It is obtained by sim- 

 ply boiling the chips in water, until ^ 1 BF mj£^jj2^&'£ 

 the inspissated juice has acquired a proper consistency. 

 The liquor is then strained, and soon coagulates into a mass. 

 It comes to this country both from Bombay and Bengal. 

 It consists chiefly of tannin, but contains also a little alu- 

 mina, which may perhaps assist in fixing the colour on 

 the cloth. 



The catechu is dissolved in acetic acid; a solution of 

 copper and sal ammoniac is added to it, and the whole 

 printed on the cloth. It is allowed to stand a few days, during 

 which the colour deepens very much, and is then worked off. 



Chrome Orange. --The dichromate jt w *3flt* *4aS* 

 of load is precipitated upon the sur- •nT" *it 

 face of cotton cloth, by printing on '- 

 it a solution of lead, and then im- 

 mersing the cloth in a hot solution 

 of a chromic salt of potash, or of { 

 lime, containing a slight excess of 





