BEATING	91
will be so heavily struck that they will be partly fibrillated and partly bruised;
the bruised portion will give up some of its particles to float in the general
mass, other particles remaining attached to the fibrillse. If the beater is run
long enough, all the fibrillse will in time be crushed into these fine particles
and become quite useless for paper-making. Therefore, our object is to stop
the action when we have most fibrillse and a good proportion of these particles
amongst them. Our engine of stuff will be fully fibrillated. These fibrillse
and many fibres partly flattened, and some cut through with fibrillated ends,
are obviously what will interlace to, make a tenacious sheet, and the finer
particles will settle into the minutest spaces between the larger fibres and still
further entwine them together.

[Bcntlcy and Jackson
fig. 29.—the neythor press
By this latter action we lose
bulk but gain strength, solidity—
shown by hardness of rattle—and
transparency. Now in the making
of our fibrillated stock into paper
our greatest difficulty lies in
getting rid of the extra water
carried by the stuff. Some is
drained out by the sucking action
of the tube rolls of the wire, some
is drawn out by the suction boxes
and suction couch. The pressure
of the press and couch rolls
accounts for a great deal. But all
the extra surfaces of the fibrillated
fibres retain some surface water, and part with it only by evaporation on the
hot drying cylinders.
It simply amounts to this: we have to dry a much greater surface area
when the stock is highly fibrillated than when the stuff is free beaten.
Again, it is a matter of great difficulty to run heavy substances, say 72 Ib.
Imperial, when using a rag stock highly fibrillated, to produce a strong paper.
The maximum strength obtainable from the stock is limited to the capacity
of the machine to extract this unwanted water on the wire before the couch
nip—i.e. the stuff must, in most cases, be let down to the chest before it has
been beaten long enough to get all the good out of its fibril.lating capacity.
To overcome this difficulty it is common mill practice to use steam heat in
the stuff and water. This is said to 'dehydrate* the stuff, but this dehydra-
tion does not reduce the strength of the paper. The increase of temperature
reduces very greatly the viscosity of water, and it is this viscosity of w$tear