The Typical Cellulose and the Cellulose Group 19 



be found fully treated of in the t'^xt-books of these arts. We can 

 only call attention to those properties which are common to the 

 group of basic oxides capable of acting as mordants, viz. (i) 

 they are all oxides of di- or polyvalent elements ; (2) they form 

 colloid or gelatinous hydrates ; (3) their salts dissociate in solu- 

 tion into acid and basic salt ; (4) they are soluble in the alkaline 

 hydrates either directly or in presence of* organic' hydroxy- com- 

 pounds. Certain of the acid oxides of the metals are also removed 

 by cellulose from solutions of these salts, but in relatively small 

 proportion ; of these the stannic compounds are most important 

 from the point of view of application as mordants. 



Amongst ' organic ' acid bodies, tannic acid is conspicuous for its 

 'affinity' for cellulose. From aqueous solutions of tannic acid 

 cotton fibre takes up as much as 7-8 p.ct. of its weight ; and the 

 process of mordanting with this compound is one ot the most 

 generally useful. 



The combinations of cellulose with colouring matter open up a 

 number of interesting problems. A colouring matter may be said 

 to dye a fibre or substance when it forms with it a Make' com- 

 pound, a lake being merely a pigment form of the colouring matter 

 in which its essential physical properties are preserved. By recent 

 investigation the properties determining lake formation have been 

 shown to be definitely correlated with the molecular constitution of 

 the colouring matter, i.e. more particularly with the nature and dis- 

 position of its chromogenic groups (NH 2 : SO 3 H, COOH, NO 2 , 

 N.OH, and OH groups). 



An excellent treatment of this subject will be found in two papers 

 by C. O. Weber in the J. Soc. Chem. Ind. 10,896, 12,650, to which 

 the student is referred. A discussion of these problems is outside 

 the scope of this work. It may, however, be pointed out that, as of 

 course the phenomena of dyeing depend upon reciprocal attraction, 

 we may confidently expect that further investigation will lead to a 

 correlation of the specific or selective attractions of cellulose for 

 colouring matters, with its molecular constitution. It should be 

 remembered that there are three factors of the problem : (i) the 

 constitution of the colouring matter ; (2) that of the substance with 

 v.hich it combines ; and (3) the condition of the colouring matter 

 in solution, and the causes which determine its transference to the 

 solid with which it is brought into contact. Of these we have pre- 



ca 



