284 Cellulose 



especially mentioned, the following may be taken as the com- 

 parative yields : 



Laboratory method. 



(Yield of dry cellulose on dry raw 



material.) 



Esparto . 50-55 p.ct. 

 Straw . . 50-55 

 Wood . . 50-55 



Papermakers methods. 



(Yield of air-dry pulp on air-dry 



raw material.) 



43-47 P'Ct. Alkali process 

 33-37 > > 



35-43 , it 



42-48 ,, Bisulphite 



It is obvious, therefore, that the cellulosic constituents of 

 the fibres are considerably attacked, and that there is an ample 

 margin for improved results in regard to quantity as well as 

 quality of the fibre produced (pulp). 



BLEACHING PROCESSES. These processes appear to divide 

 themselves into the two groups : (a) the bleaching of textiles ; 

 (b) of paper pulp. It will be evident, however, from the present 

 treatment of the subject, that bleaching is a process of purify- 

 ing a cellulose or compound cellulose from adventitious con- 

 stituents, whether mechanically mixed with the tissue or fabric, 

 or chemically united to the ultimate fibre-cellulose ; and on this 

 view of the subject bleaching treatments divide themselves 

 into (i) processes for fat purification of a compound cellulose, with 

 removal of colouring (or discolouring) matters (jute textiles ; 

 flax yarn, ' creaming ' and half bleaching process ; linen textiles, 

 part bleaching ; pulps for wrapping and coloured papers) ; (ii) 

 processes for the isolation of a pure cellulose (cotton textiles, 

 linen textiles, papermaker's cellulose). 



The bleaching process proper is the whitening or decolourising 

 process which follows such alkaline treatment as those already 

 described. The bleaching is invariably a treatment with oxi- 

 dising agents, usually alkaline ; ' bleaching powder' or calcium 

 hypochlorite is the 'staple' reagent. Other hypochlorites 

 (sodium and magnesium), obtained by double decomposition 

 from the former, are largely used, and oxidising solutions obtained 



