Each of the three pans having been charged with brine to the proper level, exhaust steam from the engines is admitted to the calandria of the first pan in which the highest temperature is maintained. The brine in this pan becomes quickly heated, and the steam given off enters the calandria of the second pan, where it serves to raise the temperature of the brine. After doing its work in the second stage, the steam is condensed, and thus creates a partial vacuum in the first pan. The atmospheric pressure being thus reduced, violent ebullition of the brine in the first pan results. The same process takes place in the second pan, owing to the calandria of the third pan acting as a condenser of the vapour and producing a vacuum. The vapour given off by the brine in the third pan is condensed by means of a jet condenser. It will, therefore, be seen that the highest vacuum and the lowest temperature exist in the third pan, while the highest temperature and lowest vacuum are found in the first pan. As the salt is precipitated it falls to the bottom of the pans. The bottom of each vacuum pan is connected with the boot of a continuous bucket elevator, which is carried in a cast-iron, water-tight casing to a level sufficiently above that of the brine in the pans to ensure that they shall be brine-sealed. The salt is delivered into waggons and the brine drainage returns to the pans. The further treatment of the salt crystals varies with the purpose for which they are required. For table salt they are subjected to grinding, but for export they are simply allowed to drain. The general aim of the Vacuum apparatus is to divide the boiling process into two stages, in order to prepare the brine beforehand by purification, and out of the purified brine to produce the purest salt possible--chiefly by boiling under atmospheric pressure--and to acquire another liquor of the highest content in medium salt. Balzberg, in his _Die Erdesalz Erzeugung_, has to admit that the process results in the most complete purification of the common salt, but in the conclusion of his critical summary of the vacuum plant, he says: “At the same time it must be admitted that a complicated machine, which only gains, at a high cost, advantages that can be achieved by more economical and simpler means is of no use in practical business. The question then arises as to whether it is necessary, for the production of domestic or table salt, to have pure chloride of sodium, and whether it pays to use complicated machinery to attain this end.” [Illustration: THE HODGKINSON PATENT SALT-MAKING PLANT]