238 BIOLOGICAL PROBLEMS 



compacted or cemented together. With such con- 

 tinuity of substance the sheet readily takes a high 

 finish. 



Blended with esparto, wood pulp makes an ideal 

 mixture for many purposes. 



There are two great divisions of wood pulp, the 

 chemical and the mechanical. The former has been 

 boiled in soda or other chemical agent to disintegrate 

 it and clear it of lignin. It forms a pulp suitable for 

 inclusion in the best papers, and, owing to the porosity 

 which its membranes acquire by the extraction of the 

 lignin, it lends itself to hydration in the beater. 



Mechanical pulp, on the other hand, has never been 

 boiled. It is formed by grinding the wood into small 

 fragments in a stream of water. It belongs to the 

 lowest order of paper-making materials, and is without 

 any capacity for felting. Moreover, as it contains the 

 unaltered lignin of the wood, papers formed of me- 

 chanical pulp rapidly undergo oxidation, becoming 

 discoloured and brittle. 



Mixed with a minimum of chemical pulp, which 

 supplies the qualities it lacks, mechanical pulp finds 

 enormous application for news and common printing 

 papers. 



Though far from being a high-class raw material, 

 mechanical pulp is good enough for the purpose to 

 which it is put, namely, to form a tissue strong enough 

 to go through the printing-press and hang together 

 till read. It is an ephemeral product, and should not 

 be regarded from any other point of view. 



At the present time an attempt is being made to 

 use sawdust as a substitute for mechanical pulp, and 

 newspapers have already been produced into which 

 sawdust enters. The application is a promising one, 

 as more sawdust is now produced in Britain than 

 ever before, in consequence of the wholesale felling 



