There are a number of types of evaporators using the same principle, although we show one only, that which is most extensively used, the well-known Swenson machine. This apparatus consists of one, two or three cells, dependent upon the economy it is wanted to attain. [Illustration: FIG. 73.--SWENSON EVAPORATOR.] =Descriptive of the Apparatus.=--The cells are numbered _1_, _2_ and _3_, beginning at the left. The rectangular box at the bottom of each cell consists of a cast iron false head or chamber at either end, in which copper flues are expanded. Steam is introduced into the chamber of _No. 1_, and the tubes surrounded with tank water. The steam in the coils causes the water to boil and the steam boiled off is passed to the steam chamber of the second cell. The tank water in cell _No. 1_ by this boiling is condensed and is passed to cell _No. 2_, where a further boiling takes place, and the concentrated liquid is passed to cell _No. 3_ for its last boiling. The auxiliary pumps shown are for transfer purposes from cell to cell, and for pumping the condensate from the steam chambers. _Vacuum._--The large pump at the right hand is the vacuum pump. The vapor withdrawn from cell _No. 3_ by the withdrawal action of the pump is condensed by the introduction of cold water in the conical enlargement shown. This condensation produces a partial vacuum, the customary gauge measurement showing 23 to 27 inches, dependent upon the quantity and temperature of the water. This vacuum continues in a lesser degree to the second cell, thence to the first and where the vacuum usually maintained is six inches. By reference to the table of boiling points the comparative temperature in each cell is ascertainable. =Cleaning the Evaporators.=--Evaporators to be effective must be clean, on the same principle as cleaning boilers produce best results. At regular intervals, say, fortnightly, they should be boiled out with a solution of 75 lbs. caustic soda in 1500 gallons of water. This will clean the flues and maintain the machine output. TABLE OF BOILING POINTS. 29 inch vacuum 79.07° Fahr. 28 inch vacuum 101.15 Fahr. 27 inch vacuum 115.06 Fahr. 26 inch vacuum 125.38 Fahr. 25 inch vacuum 133.77 Fahr. 24 inch vacuum 140.64 Fahr. 23 inch vacuum 146.78 Fahr. 22 inch vacuum 152.16 Fahr. 21 inch vacuum 157.00 Fahr. 20 inch vacuum 161.42 Fahr. 19 inch vacuum 165.42 Fahr. 18 inch vacuum 169.14 Fahr. 17 inch vacuum 172.63 Fahr. 16 inch vacuum 175.93 Fahr. 15 inch vacuum 179.03 Fahr. 14 inch vacuum 181.92 Fahr. 13 inch vacuum 184.68 Fahr. 12 inch vacuum 187.31 Fahr. 11 inch vacuum 189.83 Fahr. 10 inch vacuum 192.43 Fahr. 5 inch vacuum 202.92 Fahr. 1 inch vacuum 212 Fahr. =Storing Stick.=--The residue from the evaporators or the “Stick” as it is now called, should be pumped into a receptacle fitted with closed steam coils. The tank water vats are similarly fitted.