454 On Madder, and [Dec. 



of cotton dyed, are equally, as it were, supersaturated with 

 colouring matter, and possess a shade whiqh the dyer does 

 not intend, as is the case with the saturated combination 

 of madder purple with the alum mordant ; it is necessary to 

 determine the proportions in which the colouring matter 

 must be added to the cotton, in order to produce a definite 

 shade. This is done (after the proportion is determined in 

 which the cloth can take up the greatest quantity of colour- 

 ing matter) by using the same weight of colouring matter, 

 but different weights of cotton, perhaps the double, triple, 

 or quadruple quantity, making many trials of the colouring 

 power, and placing the pieces, not successively, but at once, 

 in the solution. By this method of proceeding I have ar- 

 rived at the result that 1 part of madder-purple, with 80 of 

 cotton, produces a saturated red. 



To determine, chemically, the difference of one colouring 

 matter from another, it is necessary to apply an excess of the 

 colouring matter to the cloth. In this way I have found that 

 madder-red is most completely distinguished from madder- 

 purple. (Rungeuses for these purposes a dyeing apparatus 

 consisting of a steam and dye boiler, with a worm and 

 refrigatory. The temperature reaches from 207°J to 209°-|). 



Constituents of Madder. — The root of madder, in reference 

 to its chemical composition, is very remarkable. It con- 

 tains seven different substances, six of which are peculiar 

 coloured compounds, and three of the latter true dyes. By 

 the following process these may be distinguished from each 

 other. Their names are derived from their properties : — 



1 . Madder-purple ^ is an orange-coloured crystalline pow- 

 der. It imparts to the cotton which has received the mor- 

 dant a deep reddish-brown purple colour, when it is em- 

 ployed in excess. If, on the contrary, the cotton is in 

 excess, the colour is a bright red. A boiling solution of 

 alum forms with the madder-purple a cherry-red solution, 

 which is not altered on cooling, if not in excess. Caustic 

 potash forms with it a fine cherry-red colour. Solution of 

 carbonate of soda forms a cherry-red solution, which is not 

 altered by potash. Sulphuric acid produces a bright red 

 colour. 



2. Madder-red^: is a yellowish-brown crystalline powder. 



* Records of General Science, vol. i. p. 15. t Ibid. 14. 



