23 
Pakt III.] Raitt : Bamboo as Material for Paper-pulp. 
conditions governing modern factory practice. The fixed charges incurred 
by a factory for interest and depreciation of plant, salaries, wages, etc., 
bear so high a proportion to the total cost of pulp per ton, that the amount 
of output becomes of supreme importance. The greater the production, 
the less the ton cost for these items, and it may easily be of so much im¬ 
portance that it may pay to sacrifice chemicals, or even yield, in order to 
save time and thus increase the output. To take a concrete case :—it is 
conceivable that a yield of 40 per cent, obtained at a cost of 20 per cent, 
of NaOH in 6 hours may pay better than one of 44 per cent, obtained 
by the use of 18 per cent, in 12 hours. Our best guide on this point is 
modern factory practice which declares that 8 hours is the outside limit 
which can be allowed for digestion inclusive of the time required for filling 
and discharging the digester, which means 6J or 7 hours’ actual digestion, 
and the further that this can be reduced, without unduly raising the 
chemical consumption or reducing the yield, the better. Temperature 
therefore is the only factor permitting of any considerable degree of lati¬ 
tude. Our present problem therefore is, to take a specimen case :—what 
amount of NaOH, temperature and time are necessary to secure complete 
but bare digestion (we are still at this stage leaving out of account the 
bleaching effect of an excess of NaOH) of crushed B. polymorpha, the 
minima being 15*79 per cent. NaOH, 150° of temperature, and 3 
hours, the maximum time permissible being 6J hours. 
29. It is not necessary to detail the long series of experiments which 
were required to settle the above query, but it may be said that there was, 
with liquors not exceeding 12° Tw., incomplete digestion in 6J hours at 
153° but satisfactory digestion at 162°. The destruction of fibre up to 
that limit scarcely concerns us as the necessity for complete digestion is 
paramount. Between 162° and 170° the additional loss was small and 
might be considered compensated for by a saving of one hour and a slight 
improvement in colour. At 177° the loss was serious, and at 183° extra¬ 
vagant. It was, however, found that the advantages of high temperature 
in more rapidly overcoming penetration resistance and improving colour 
could be obtained, without incurring the penalty of loss of fibre, by con¬ 
ducting digestion at 177° for not more than 1 hour and then reducing to 
162° for the remainder of the time. This resulted in the saving of 1J hours, 
digestions thus managed being as good in 5 hours as others conducted at 
a uniform temperature of 162° for 6| hours. It is reasonable to assume 
that the penetration of the tissues takes time and that fibre destruction 
is not likely to take place until the pectous and ligneous matter is at least 
thoroughly softened, and it would seem that this part of the process 
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