But the outstanding characteristic of the vacuum system is the increased yield of fat forthcoming. No mechanical system, whether it be pressure or high-speed whizzing, can extend completely satisfactory efficiency results. As is well known, the fat entering into the constitution of animal matter is contained in myriads of minute cells which are surrounded by tissue. The walls of these cells are exceedingly elastic and of prodigious strength. They may be compressed to an inordinately intense degree in a press, or distorted and stretched by recourse to centrifugal action without breaking. It is this circumstance which reacts against a high recovery of fat by recourse to pressing and whizzing because the cells cannot be induced to burst. When a vacuum is applied a totally different result is recorded. The application of heat causes the fat and air within the tiny cells to expand, and in this manner the walls of the cells become distended to the limits of their elasticity. The removal of the surrounding air within the vessel by the vacuum pump completely upsets all equilibrium. The air pressure within the cells is higher than that applied from without, and consequently there results an accentuated expansive effort within the cells. But the tissue has already been stretched to its utmost limit, and so being unable to withstand the increased strain imposed collapses, thus releasing the imprisoned air and fat. Under the vacuum process the disruption of the fat-carrying cells is complete, and this explains why an augmented yield of fat is obtained by this method. Under the open steam vacuum process the actual practice is to apply the vacuum three times at intervals during the operation. The first application serves to remove the obstructive air to facilitate and expedite cooking of the contents. The second brings about the disruption of the cells and the release of the fat which they contain. The third application of the vacuum, which is effected towards the end of the process, effects the withdrawal of the foul vapours arising from the digesting operation and their discharge into the fire. Owing to the steam being admitted to the digester and being allowed to come into direct contact with the mass, the residue upon withdrawal is wet. The grease, which has been rendered fluid in the process, has escaped from the digester through a suitable draining pipe into a tank where settlement and clarification are carried out. But all the grease cannot be recovered in this manner. A certain proportion, notwithstanding the disruption of the fat cells, is held up in the mass and can only be recovered to an appreciable degree by submitting the residue to treatment in a press. In this way the greater part of the remaining fat suffers expulsion and recovery. The wet cakes upon removal from the press then have to be dried and disintegrated.