The action of the boiler is as follows:—The steam passing through the pipe A heats the liquor that has drained from the grass through the perforations in the false bottom, and forcing it up the wide pipe C, causes it to strike against the dome or bonnet D, and distribute itself again over the grass. {92} This is technically known as “vomiting.” In this way a constant circulation of liquor is kept up. [Illustration: FIG. 24.] The boiler is emptied by the door H and the liquor is run off by the cock I. The boilers are usually supplied with a safety valve E, and also with a pipe for letting off the steam when the boiling is finished. The boiling generally takes from four to six hours. The quantity of soda necessary depends upon the nature of the grass, Spanish requiring less than African. {93} Routledge, to whom the introduction of esparto as a paper-making material is due, gives 10 per cent. as the proper quantity. The pressures employed vary from 5 lb. to 45 lb. There has been a growing tendency of late to employ the higher pressures for reasons already stated. [Illustration: FIG. 25.] Of late years great improvements in the form of the boiler have been introduced. Fig. 25 is an illustration of Roeckner’s Patent Boiler. The vomit pipe is outside the boiler. Steam entering by the cock D, forces the liquor up the vertical vomit pipe and distributes it over the grass. The pipe K is used for heating the liquor at the commencement of the operation by means of waste steam. F F are gauges for indicating the height of the liquor. The grass is put in by the opening G. The pipes A B and C are for the supply of {94} steam, strong lye and water. The boiled grass is discharged by the circular door E. The boiler holds three tons of grass and the boiling is completed in about 2 1⁠/⁠2 hours, the pressure being about 35–40 lb. per square inch. It is claimed that by its use a saving both of time and soda is effected. [Illustration: FIG. 26.] Sinclair’s Patent Boiler is shown in Fig. 26. The vomit pipes, of which there are two, are made of thin steel plates {95} riveted to opposite sides of the boiler. The liquor drains through the perforated false bottom, and is then forced up the vomit pipes above the perforated plates through which it is distributed over the grass in a number of fine jets. The boiler is charged by the door _e_ and emptied by _j_. The small cock _a_ is used as a blow-through cock; the opening _b_ is used for blowing-off steam when emptying the boiler; _c_ for letting in caustic soda lye; _d_ for water. Steam enters by the small branch of the T pipe at the bottom of the boiler, the other, _g_ being used for running off the waste liquor. A boiler holds from two to three tons of grass, and a boiling is completed in about two hours at a pressure of 40–50 lb. per square inch.