PRODUCTION OF DYES 329 



of the fibre, while the mineral acid is left in the bath. This may 

 be expressed diagrammatically as follows : 



W< 2 2 H +BHC1 give 



Wool Dye Dyed Mineral 



fibre hydrochloride wool acid 



In a similar manner the process of dyeing by acidic colours, 

 such as picric acid, may be represented. In this case a com- 

 bination occurs in which the colour acid, HA, unites with the 

 ammonia residue of the fibre, thus : 



Dyed wool. 



Such has been the hitherto-accepted theory of the dye-bath. 

 During recent years, however, the study of the phenomena of 

 surface attractions, not usually regarded as chemical in nature, 

 has led to discoveries which require a modification in such 

 theories. 



It has long been known that many substances such as charcoal 

 have the property of withdrawing colouring matters from 

 solution, and the fact has long been turned to practical account 

 in the refining of sugar. The syrups from which white sugar is 

 to be obtained are filtered through thick beds of bone charcoal, 

 where the brown uncrystallisable substances present are adsorbed 

 into the substance of the charcoal, while the sugar is retained 

 in solution. This power possessed by charcoal seems to be 

 connected with the extent of surface of the particles of which 

 it is composed, and it is shared to a greater or less extent by 

 other substances which can be got into a fine state of division, 

 notably by metals such as platinum and palladium, metallic 

 oxides and hydroxides such as alumina, and even by fine sand 

 and powdered glass though in an inferior degree. 



The fibres of silk, wool, and cotton consist of substances 

 possessing more or less the character of colloids (Chap. XIII). 

 They are capable of taking up considerable quantities of water, 

 not, however, in any definite proportion corresponding to the 

 formation of chemical compounds. In the imbibition and 



