The transference of heat is also much influenced by the viscosity of the liquor being evaporated; the greater the viscosity, the lower the coefficient of heat transmission. Unfortunately for this process of evaporation, gelatine sols are exceedingly viscous, and thus the difficulty in obtaining a concentrated sol is thus greatly enhanced. The transference of heat is often greatly hindered by incrustations of the tubes, which incrustations generally conduct heat very badly. Thus the relative heat conductivities of copper and chalk are as 1000:5. The amount of heat transferred is of course determined also by the area of the heating surface. The amount of evaporation needed thus determines the number of tubes (of standard size) in the evaporator, and thus the capacity of the machine. An evaporator should have its heating surface area chosen with a view to the duty required of it. In practice the working of an evaporator is often not a very difficult matter, and large numbers of machines are operated by unskilled labour. Troubles generally arise from inconstant steam pressure, incrustation, leakages of air, which reduce the vacuum, the temperature head, and hinder heat transmission. For the evaporation of gelatine liquors the Yaryan, the Kestner, and the Blair-Campbell film evaporators are the most widely used. The velocity of the liquor through some of these machines is so great that occasionally no vacuum is used. The temperature obtained is high (200° F.), but the time is very short, if rapid cooling of the evaporated liquor is arranged. REFERENCES. "Evaporating, Condensing and Cooling Apparatus," by E. Hausbrand. Scott, Greenwood & Son (1916 Ed.). "Evaporation," by E. Kappeschaar. Norman Rodger (1914). "Evaporation in the Chemical Industry," by J.A. Reavell, M.I.Mech.E., _J.S.C.I._, 1918, April 11th. "Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59. "Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29. "Notes on Condensing Plant," J.M. Newton, B.Sc., _J. Junior Inst._ Engineers, Aug., 1912. SECTION VII.--COOLING AND DRYING The conversion of a gelatine sol into cakes of gelatine has been much simplified by the advent of the evaporator. Before this machine was used much trouble was experienced with putrefaction, and in hot and thundery weather, especially on the Continent, it was often necessary to suspend operations. Evaporation has, however, materially contributed to the possibility of rapid and satisfactory cooling and drying.