236 TIMBEES AND THEIR USES 



tion of the pulp has been studied by Chve and 

 Thickens, who state that under similar con- 

 ditions the strength of the pulp increases with 

 the power expended per ton in the grinding. 

 The qualities of mechanical pulp, unhke those 

 of the cellulose fibres, cannot be modified sub- 

 sequently in the beating process, so that the 

 paper making is entirely dependent on the con- 

 ditions adopted in grinding. 



" At the present time the main varieties of 

 mechanical wood pulp are prepared, viz. ' cold- 

 ground ' and ' hot-ground.' Cold grinding is 

 effected under moderate pressures between wood 

 and stone, the pulp being washed away by 

 copious streams of water. In hot grinding the 

 pressure of the wood against the stone is ex- 

 tremely high and the stream of water reduced 

 to a minimum, so that a very high temperature 

 is developed at the surface of contact. Hot 

 grinding is more economical than cold grinding, 

 and is stated to yield a finer and larger pulp 

 than cold grinding, but cold-ground pulp pos- 

 sesses a quality known to the paper maker as 

 ' wetness ' or hydration in a high degree. 



" A. distinct variety of mechanical pulp is 

 manufactured from logs which have been 

 thoroughly softened by steaming in digesters 

 under pressure. This is known as ' brown 

 mechanical ' or ' leather-board ' pulp. The 



