20 
Indian Forest Records. 
[Vol. Ill 
So far as analysis can teach us, the figures given in the last column 
represent the irreducible minima with which bare but complete resolu¬ 
tion can be accomplished under nearly perfect conditions of resistance. 
Anything in excess of these amounts which may be found necessary will 
be due to conditions of material, or digestion, other than the non-cellulose 
contents of the material. 
SECTION C. 
Digestions. 
23. The methods of digestion applicable in factory practice for the 
isolation of cellulose by bringing into solution the other constituents of 
the material are three in number and are known as the Sulphite, the 
Soda and the Sulphate processes. The liquor employed in the first of 
these carries in solution bi-sulphite of lime,—or when dolomite limestone 
is used, a mixture of the bi-sulphites of lime and magnesia,—and free sul¬ 
phur dioxide gas. The soda process depends on the saponifying effect of 
sodium hydroxide (caustic soda). An important feature of this process 
is the recovery of the reagent from the spent liquors, by evaporation 
and calcination, in the form of carbonate of soda. This is regenerated 
to caustic soda by boiling with quicklime and re-used. It is thus pos¬ 
sible to recover 80 to 90 per cent, of the whole. The loss is made up with 
fresh carbonate added during the causticising process. The sulphate 
method is a modification of this ; the loss on recovery being made up 
with sulphate of soda which is changed during calcination to sodium 
sulphide, and the ultimate liquor contains as its active ingredients, caus¬ 
tic soda (NaOH) and sodium sulphide (Na 2 S). 
24. The advantages possessed by the sulphite process over the soda 
are, (a) that when conditions exist favourable to 
Sulphite d ges ons ^he manufacture of the liquor it is less costly, ( b) 
it exercises a less destructive effect on the fibre, so its yield of cellulose 
is slightly higher, and (c) it does not, like soda, degrade the solubles of 
the material to a brown colour which stain the pulp. The latter is there¬ 
fore of a fairly good yellowish white colour in the unbleached state and 
can be used for many purposes (such as the cheaper grades of newspaper), 
without further bleaching. But this advantage is largely lost where 
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