Cellulose 48·25 Water 9·38 Aqueous extract 10·19 Pectous matter 26·39 Fatty matter 2·07 Ash 3·72 ------ 100·0 ------ Yield of dry cellulose obtained in actual practice from good raw material 45 to 48% By boiling the esparto with caustic soda under pressure for a stated time, the non-fibrous constituents are removed, leaving the cellulose in a more or less pure form according to the severity of the chemical treatment. [Illustration: FIG. 20.--An Esparto Duster.] In practice the grass is packed tightly into upright stationary digesters and a definite quantity of caustic soda solution added, the amount of chemical used being equal to 15-18 per cent. of the weight of grass packed into the digester. The form of digester almost universally employed is that known as the Sinclair's "vomiting" boiler, which is constructed so that a continuous circulation of the liquid is maintained by means of what are called "vomit" pipes. These are fitted to the sides of the digester in such a manner that the caustic soda solution circulates from the bottom of the digester, up through the "vomit" pipes, and is discharged downwards upon the contents of the boiler through a perforated plate fixed in the upper part of the digester. The requisite quantity of caustic soda solution is placed in the digester, and steam admitted into the bottom of the vessel while the grass is being thrown in. In this way a much larger weight of grass can be boiled at one operation, since the bulk is greatly reduced when the grass has become thoroughly soft and wet. [Illustration: FIG. 21.--Sinclair's "Vomiting" Esparto Boiler.] When the boiler is loaded the inlet is closed up and steam turned on to the full pressure of about 40 or 50 lbs., this being maintained for a period of about four hours. The non-fibrous constituents of the esparto are gradually dissolved out by the caustic soda, and when the operation is completed the black liquor is run off from the digester into large store tanks, and the esparto grass which remains in the digester is then completely washed until the soda is almost entirely washed out. [Illustration: FIG. 22.--A Porion Evaporator.]