_Recovery of Spent Liquor._--As it is possible to recover 75 to 80 per cent. of the soda originally used in digesting the esparto, the washing of the boiled grass is conducted on scientific principles in order to ensure a maximum recovery of soda at a minimum cost. The recovery is effected by evaporating down the black liquor, together with the washing waters, to a thick syrupy mass, which can be burnt. The organic and resinous constituents of the esparto which have been dissolved out by the caustic soda, forming the soluble soda compounds, ignite readily, and during combustion the organic soda compounds are converted more or less completely into crude carbonate of soda. It is obvious, then, that the cost of recovery depends mainly on the quantity of weak washing water which has to be evaporated. Consequently methods are devised by means of which the grass is thoroughly washed with as little water as possible, and some of the methods are very ingenious. The spent liquors and washing waters are evaporated to a small bulk in a vacuum multiple effect apparatus, and the thick liquid mass obtained by evaporation is burnt either in a rotary furnace or on an ordinary hearth. Every precaution is taken to effect this operation with a minimum quantity of coal. The burning off of this mass results in the formation of a black substance which is taken away from the furnace and allowed to char or slowly burn until the impure white soda ash, or carbonate of soda, is obtained. Two systems of recovery are in general use, which deserve a brief notice:-- _Direct Evaporation._--The liquors may be evaporated to a small bulk ready for incineration by treatment in long shallow pans or furnaces, the heat necessary for the process being obtained mainly from the combustion of the thick concentrated liquor. The most familiar type of this form of apparatus is the Porion evaporator. [Illustration: FIG. 23.--Scott's Multiple Effect Evaporator.] The combustion of the concentrated liquor is started by a coal furnace at one end of the apparatus. The thick viscous mass catches fire and burns with a fierce flame, and the heat is utilised in evaporating the weaker liquors which flow continuously through shallow brick troughs, the surface of which is freely exposed to the heat and flames from the hearth where the organic soda compounds produced in the boiling of esparto are being incinerated and converted into soda ash. Under suitable conditions this evaporator is most economical in its results. It can be erected cheaply, and when all the heat is fully used in every possible direction it can be worked at a low cost compared with the more modern multiple effect evaporators.