326 CHEMICAL DISCOVERY AND INVENTION 



hyposulphite of sodium specially manufactured for the purpose. 

 The result in any case is a yellow liquid, which on exposure to 

 the air absorbs oxygen and reproduces the blue. Hence for 

 dyeing cotton the cloth is immersed in the alkaline liquid, and 

 when saturated it is exposed to the air and j the blue is deposited 

 in the insoluble form within the fibre. 



Some of the derivatives of indigo produced by the introduction 

 of bromine into the molecule are important dye-stuffs which are 

 known in the trade as Ciba dyes. 



In this connection an interesting discovery has resulted from 

 the modern investigation of the purple dye extracted as in ancient 

 times, from a species of murex, a mollusc found in the Mediter- 

 ranean, and generally referred to as Tyrian purple. It is now 

 known to be dibromindigo with the following formula : 



NH 



The next dyestuff which may be described is very different in 

 constitution from indigo, but is interesting not only on account 

 of its great practical importance, but because it has a very 

 similar history. The substance referred to is the red colouring 

 matter of the madder root. 



The madder plant is a herbaceous perennial, Rubia tinctorum 

 (Nat. Ord. Stellatce), very nearly allied to the common goose 

 grass or cleavers of our hedges. The R. peregrina, which is re- 

 garded by Hooker (Bentham's British Flora) as probably a 

 mere variety of R. tinctorum, is found in the southern and western 

 parts of Britain and of Europe. The use of the root as a dye 

 can be traced to very ancient times, as it is not only mentioned 

 by Pliny, but the red dye-stuff has been recognised in the 

 mummy cloths of Egypt. It has been cultivated for centuries 

 in the Levant, and in 1766 was introduced into the south of 

 France by Jean Althen, to whom a statue was erected at Avignon. 



