190,237. British Thomson - Houston Co., Ltd., (General Electric Co.). Sept. 14, 1921. Film evaporators.-A method of preparing tungsten oxide in a dried and powdered condition consists in providing a supply of the washed precipitate mixed with liquid of a neutral character to form a thin paste, depositing the paste upon a heating-surface in a thin layer, and removing the dried layer from the surface so as to break up the layer into a powdered condition. According to one set of means for carrying out the invention, the precipitate flows from the settling tank into a trough 11, Fig. 1, having reciprocating agitators 12, and through an adjustable opening in the side of the trough and a shoot 24 into a pool formed by a dam 30, Fig. 2, which is substantially in fluid-tight contact with the steam-heated rotary drum 28. A thin film of paste adheres to the drum and after being dried is removed by a reciprocating brush 56 in the form of a powder, which falls into a receptacle 70. The drum is rotated through worm-gearing from a shaft 22, which may be rotated in opposite directions by operating the clutch 82 through a lever 85 so as to engage with either of the gears 76, 77; these are driven through a gear-box 72 from an electric motor 71. The brush is built up of elements 57, Figs. 6 and 7, secured to a flexible back 58 enclosed within a block 59, which slides in a guide 60 carried by the frame 39. The brush is held resiliently against the surface of the drum by spring-pressed rollers 61 secured to a member 63 carried by the frame. Both the agitators in the trough and the brush are reciprocated through cranks and rods from the shaft 22. The speed of the brush can be varied by change-gearing.