CASEIN : ITS OKIGIN, PREPARATION AND PROPERTIES. 15 



broken, by the material under treatment when in large lumps ; 

 and they also break the material down too small in some 

 cases. The rotary movement of the cylinder or of the shaft 

 not only stirs up the material, and thus greatly accelerates 

 drying by presenting fresh surfaces for evaporation, but also 

 gradually moves the material onward in the cylinder. The 

 operation, therefore, proceeds continuously, in the same way 

 as in a drying flue, though more rapidly on account of the 

 constant stirring, and yields a product of perfectly uniform 

 dryness throughout. The onward movement of the .material 

 is effected in various ways, either by means of a rotating 

 shaft (inside the cylinder) fitted with spirally mounted arms, 

 and therefore acting like a worm conveyor, or else by mount- 

 ing the drum with one end higher than the other. The 

 latter effect may be also obtained by the use of a conical 

 casing, the shaft being mounted horizontally, though the 

 cylindrical form, with the shaft tilted at an angle with the 

 horizontal, is preferable, on account of lower cost and greater 

 ease of manufacture. In this system the central shaft and 

 arms are unnecessary, the drum being actuated by a toothed 

 crown round the outer edge, driven by worm or cog gearing. 

 Worm gearing is preferable, being more easily fitted and 

 giving the requisite slow movement without any troublesome 

 reducing gear when the shafting is run at the ordinary speed. 

 In view of the small power needed to drive the cylinder, no 

 objection on the score of useful effect applies to the use of 

 worm gearing. 



The interior of the cylinder must be fitted with blades, 

 which lift up the material under treatment and allow it to 

 fall down again slowly and in small quantities, so that each 

 particle is repeatedly exposed to the surrounding warm air. 

 At the same time, the material is always lifted perpendicular 

 to the axis of the drum and falls vertically, thus describing a 

 spiral line, the pitch of which depends on the slope of the 



