Of these there are two chief forms, the first being a tank or pan formed of large pieces of slate, with the joints made with clay, and surrounded with a mud wall.  The whole is covered with an arch or vault and is filled with the brine, which is then evaporated by surface heat, the fire being placed at one end and the flue at the other.

The other form is very curious and interesting, and is almost identical in its principle of construction with the pan I have referred to as figured in Agricola, only in this case the materials are very different, being, instead of wood and iron, nothing more than clay or mud.

It was described officially by the Japanese, in their publications at the Philadelphia Exhibition in 1876.  The Japanese description of this apparatus is highly interesting.  It is as follows:—­

A low wall is built, enclosing a space of about 13 feet by 9 feet, the bottom forming a kind of prismatical depression, 3 feet deep in the centre line.  An ashpit, 3 feet deep, is then excavated, starting from the front wall, and extending about 4 feet into this depression at its deepest place; it communicates with the outside by a channel sloping gradually upwards, and passing underneath the front wall.  The ashpit is covered by a clay vault, with holes in its sides, so as to establish a communication between the ashpit and the hollow space under the pan.  This vault is used as a fire grate, the fuel (brown coal and small wood) being inserted by the fire-door in the front wall.  The air-draught necessary for burning the fuel enters partly by the fire-door, partly through the ashpit and the openings left in the vaulted grate.  Through these same openings the ashes and cinders are from time to time pushed down into the ashpit, for which purpose small openings are left in the side-wall of the furnace, through which the rakes may be introduced.  A passage in the back wall supporting the pan leads off the products of combustion and the hot air into a short flue, sloping upwards, and ending in a short vertical chimney.  At the lower part some iron kettles are placed in the flue for the purpose of heating the lye before it is ladled into the evaporating pan.

With reference to the pan, it is made in a way that requires a great deal of skill and practice.  In the first place, beams reaching from the one side to the other are laid on the top of the furnace walls, and are covered with wooden boards, forming a temporary floor.  Two or three feet above this floor a strong horizontal network of poles of wood sustains a number of straw ropes, with iron hooks hanging down, and of such a length that the hooks nearly touch the wooden floor.  The floor is thereupon covered with a mixture of clay and small stones, 4 to 5 inches thick, the workman being careful to incrustate the iron hooks into this material.  It is allowed to dry gradually, and when considered sufficiently hardened, the wooden beams and flooring are removed with the necessary