A great stride forward was made by Howard's invention of the Vacuum Pan. This made it possible to undertake concentration at much lower temperatures, a most important improvement in the case of gelatine and other organic matters easily damaged by heat. The process, however, was still slow, and the sol exposed to heat for a long time, as must be the case when evaporation takes place in bulk. These disadvantages were still fatal to the production of the highest-grade gelatine. There were also the practical difficulties of entrainment ("blowing over"), in which parts of the sol were carried away by the escaping vapour, and also of "incrustation" which so rapidly reduces the heating efficiency and evaporative capacity of the machine. The vacuum pan, however, presented two decided advantages--evaporation at a low temperature, and, as a corollary, the possibility of utilizing exhaust steam to attain this temperature. Whilst the vacuum pan was a satisfactory machine for many branches of chemical engineering, the problem of evaporation was still unsolved for gelatine liquor because of the "stewing" involved, until the advent of the "film evaporator," which dealt with the liquor not in bulk, but in a continuous stream. In this way the product was only exposed to heat for a comparatively short time. Many evaporators of this type came into being, and rapid improvement was made in the constructional details. The film evaporators retained usually the advantage of evaporation _in vacuo_, so that it was now possible to evaporate gelatine sols by exposure for a short time to a comparatively low temperature. Of this type of evaporator, the Lillie, Yaryan, Schwager, Claassen, Greiner, Blair Campbell, and the Kestner machines are well-known examples. A further advance in solving this problem was the application of the principle of multiple-effect evaporation. The vapour driven off during evaporation possesses of course many heat units, and is of very considerable volume. In multiple-effect evaporators this vapour is used to work a similar evaporator, and the evaporated liquor passes immediately into what is practically a second machine, and is further evaporated by the heat from the vapour just driven from it. Such an arrangement would be termed a double-effect evaporator. The vapour from the second effect may of course be similarly used to operate a third effect, and the vapour from this to work a fourth effect, and so on. Thus, we may have triple effect, quadruple effect, etc., even up to octuple effect. The great advantage of multiple-effect evaporation is in the saving of costly steam. Reavell gives the following figures to illustrate the economy thus obtained:-- WATER EVAPORATED PER 100 UNITS STEAM.