hard paper is very similar to that employed with esparto, but the conditions of boiling are more severe on account of the presence of compound celluloses in straw, which require to be treated differently to those found in esparto. Barley, oat, wheat and rye straws all yield good pulps, which differ slightly in quality and working properties.

The initial operations of dusting, removing dirt, weeds and other foreign matter are practically identical with those used in the treatment of esparto. The boiling, however, is seldom carried out in an ordinary upright stationary boiler, but in rotary digesters, spherical or cylindrical in shape, because, with straw cut up into short pieces by means of a chaff-cutter, the mass of heavy wet stuff does not permit of the free circulation of caustic soda by the vomit of the Sinclair boiler.

If, however, the straw is boiled without being cut up, it is possible to use the upright stationary digester.

The straw is boiled for about six to eight hours at pressures varying from 70 to 90 lbs. with a solution containing caustic soda equal to 18 or 20 per cent. of the weight of straw treated.

Washing. The washing of boiled straw demands some care, because the pulp contains a large proportion of very short small fibres which escape with the washing water when an ordinary drum-washer is used. The greatest yield of paper-making fibre, both in the case of esparto and of straw, is obtained when the washing system is so arranged that the pulp remains stationary in a suitable tank, and is cleansed by a continuous flow of clean water.

Bleaching, &c. The final operations of bleaching, washing the bleached pulp, and producing the sheets of wet pulp for further use, resemble those applied to esparto. The quantity of bleach required entirely depends on the extent to which the straw has been boiled, since a well-boiled pulp may only require a small percentage, whereas an insufficiently digested straw will consume a larger proportion. Eight to twelve per cent. of bleaching powder is considered sufficient.

The table on pages 48 and 49 shows the conditions under which the various raw materials commonly used for paper-making are converted into pulp or "half-stuff."

Beating.

The process to which all bleached pulps or "half-stuffs" are submitted as being the next stage towards conversion into paper is known as "beating," an operation effected in the beating-engines or "beaters."

This term arises from the fact that, prior to the invention of a special machine based on the idea of rotating cylinders provided with steel knives to disintegrate rags, the bleached rag pulp was treated in a kind of mortar by means of

PROCESS.

RAGS.

TABLE V.-Statement respecting the Manufacture of Half-stuff.

ESPARTO.

STRAW.

SULPHITE WOOD.

SODA WOOD.

WASTE FIBRES.

a tight-fitting stamp. The complete disintegration of the half-stuff into fine pulp was thus accomplished by actual beating, there being an entire absence of any cutting action; but the process was very long and costly, though the paper obtained possessed remarkable strength and durability. An interesting account of this process will be found in a lecture read by Dr. Arnot before the Society of Arts in 1877.

Since the invention of the first beating-engine, constructed with a revolving cylinder fitted with steel knives about the year 1800, by the Dutch, many types and modifications of beaters have been introduced. In spite of the many years which have elapsed since the introduction of the "Hollander," as it is called, the original form of this early beating engine is found most suitable for the

=

B

A

Fig. 37. Diagram illustrating Effect of altering relative Position of
Beater-knives on the Roll and Bed-plate. (Klemm.)

A. Knives in close contact, producing a cutting action.

B. Knives further apart, producing a rubbing or tearing action.

proper treatment of rag half-stuff, the later machines having certain advantages for other pulps only.

The Hollander" Beating-engine. This machine is an oval-shaped trough, similar in appearance to the "breaker," having semicircular ends, and divided into two channels or compartments by a "midfeather," which extends down the centre of the trough, but does not reach the full length from end to end. The beater-roll is constructed of such a size that it operates in one of the channels, revolving at a given speed in more or less close contact with a bedplate fitted with stationary knives fixed to the bottom of the channel. The floor of this particular channel slopes gradually upwards towards the beaterroll, and then falls suddenly behind the roll, the curve of this "backfall" being similar to or concentric with the curve of the roll. The beater is thus in many respects closely allied to the breaker, but it differs in certain important