From the time of the introduction of the open-pan system in Cheshire, until the beginning of the present century it was found impossible, owing to the nature of the furnaces employed in the process, to maintain a sufficiently high and uniform temperature to produce salt which, without grinding, is marketed as finest table salt, or to make more than 2 tons of salt from the consumption of 1 ton of coal. Experiments for the purpose of economizing fuel appeared destined to perpetual failure, and the hand-stoking of the furnaces entailed so many variations of temperature that the production of salt crystals of uniform size was impossible. Then, within the same decade, two processes were invented which, between them, solved the problems that had hitherto eluded all the efforts of the scientist, the engineer, and the practical salt-man. In order to understand the advantages secured by the operation of the Vacuum System, which comes to us from the United States, it must be remembered that, under atmospheric pressure, brine boils at a temperature of 226° F., whereas in a vacuum of 28 in. mercury, the boiling temperature is reduced to about 100° F. It will thus be seen that evaporation _in vacuo_ renders it possible to use multiple effect apparatus without causing unduly high pressure in the first vessel, and it has this further advantage, that the low-pressure steam, in passing through the evaporation gives up its latent heat, whereas if the steam went to the condenser direct from the engine, the heat employed in the steam engine would be only the difference between the heat contained in steam at 170 lb. and the steam at 5 lb. pressure. By multiple effect evaporation, a great economy in the amount of steam required is effected. Between the evaporation of brine and that of other liquors, the chief difference to be noted is that in the multiple effect system, each pan or unit is supplied with its brine independently of the others, and graining goes on in the pans, whereas in concentrating other liquors the pans are fed from the first to the second and from the second to the third. The removal of the salt from each pan has, therefore, to be arranged for. The method of working a triple-effect plant may be briefly described as follows--