Concentration in open pans has now been superseded by evaporation _in vacuo_. The subject of the gradual development of the modern efficient evaporating plant from the vacuum pan, originated and successfully applied by Howard in 1813 in the sugar industry, is too lengthy to detail here, suffice it to say that the multiple effects now in vogue possess distinct advantages--the greatest of these being increased efficiency combined with economy. The present type of evaporator consists of one or more vessels, each fitted with a steam chamber through which are fixed vertical hollow tubes. The steam chamber of the first vessel is heated with direct steam, or with exhaust steam (supplied from the exhaust steam receiver into which passes the waste steam of the factory); the treated lyes circulating through the heated tubes is made to boil at a lower temperature, with the reduced pressure, than is possible by heating in open pans. The vapour given off by the boiling liquor is conveyed through large pipes into the steam chamber of the second vessel, where its latent heat is utilised in producing evaporation, the pressure being further reduced, as this second vessel is under a greater vacuum than No. 1. Thus we get a "double effect," as the plant consisting of two pans is termed. The vapours discharged from the second vessel during boiling are passed through pipes to the steam chamber of the third vessel (in a "triple effect"), and there being condensed, create a partial vacuum in the second vessel. The third vessel may also be heated by means of live steam. The vapours arising from the last vessel of the evaporating plant, or in the case of a "single effect" from the vessel, are conveyed into a condenser and condensed by injection water, which is drawn off by means of the pump employed for maintaining a vacuum of 28 inches in the vessel. In the most recent designs of large evaporative installations, the vapours generated from the last vessel are drawn through a device consisting of a number of tubes enclosed in a casing, and the latent heat raises the temperature of the treated lyes proceeding through the tubes to supply the evaporator. It will thus be observed that the object of multiple effects is to utilise all the available heat in performing the greatest possible amount of work. Special devices are attached to the plant for automatically removing the condensed water from the steam chambers without the loss of useful heat, and as a precaution against splashing over and subsequent loss of glycerine through conveyance to the steam chamber, dash plates and "catch-alls" or "save-alls" of various designs are fitted on each vessel.