328,634. Treschow, M. Feb. 1, 1929. Ball and like mills.-The material to be ground in a tube mill is exposed to cooling at suitable points of the mill by being passed along one or more heat-exchanging members, which partake in the rotation of the mill and through the wall or walls of which the heat of the material is transmitted to a cooling medium flowing within the members. The mill 1 is provided with an intermediate chamber 8 in which are cooling tubes 9 connected by an inlet pipe 18 to an annular space 16 and an outlet pipe 19 to an annular space 20, the annular spaces being formed in a fixed casting 17 having a fluid-tight connection with the outlet trunnion of the mill. The annular space 16 is connected by a pipe 15, valve 26, and pipe 14 with a tank 12 and the annular space 20 is connected by a pipe 21, pump 22 and pipe 23 to the same tank. The cooling medium is drawn through the pipes 9 by the pump so that in the event of a leak in the pipes, air is drawn in, and flooding of the mill prevented. The tank 12 is provided with an overflow 13 and a supply pipe 23, 24, a valve 25 connecting the pipes 23 and 15 so that the cooling medium may be supplied direct to the circulating system. The coils 9 are out of contact with the grinding bodies, but small material is inserted in the chamber 8 to clear the pipes of adhering ground material. In existing mills, an annular chamber 28, Fig. 3, divided by a partition 29, and provided with coils 9 encircles the mill in the neighbourhood of the coarse and fine grinding chambers. The partition 29 separates the chamber 28 into two compartments which are in communication with the mill through holes 31, 32 respectively, the mill being divided between the holes by a partition 33 so that the ground material from the first grinding chamber passes through a sieve 37, holes 31, both compartments in chamber 8 and holes 32 to lifting vanes 36 which discharge the material into the second grinding chamber. The helical formation of the pipes acts as a conveyer for the material or a separate conveyer device may be employed. The inlet and outlet pipes may be connected as previously described. In a modification the tube mill may be jacketed, the grinding surface of the tube being provided with hollow projecting ribs through which the cooling medium flows. In a further modification, instead of the pipes 9, Fig. 3, forming the cooling medium, a closed jacket, through which air circulates, is arranged to surround the chamber 28.