The stack is built up as follows: - First, a layer of ashes, upon which is placed a layer of spent tan of about 3 feet in thickness. In the older Dutch method horse-dung was used, but this is open to some disadvantages which will be pointed out presently; the use of tan was introduced in England so that this modification of the Dutch process is sometimes spoken of as the English method. This layer of tan is pressed down very firmly and is evenly spread ; on it is placed a layer of the pots, which layer is kept at a distance of about 6 inches from the sides of the shed. In some works the outside rows of pots are made of larger size than the others, so as to act as supports for a layer of flooring boards. In other places the pots are all of one size and wooden supports for the boards are provided. ,On the top of the pots is placed a layer of lead buckles or gratings (Fig. 5). These are placed face to face in a layer of about 3 to 5 inches thick; above these comes the layer of flooring boards, a space of about 6 inches being left between them. On the top of the boards another layer of tan, then a layer of pots, then a layer of gratings, then another layer of boards, and so on until the stack is completely built up. The number of sets of layers varies from seven to eleven. The doorway through which the filling is done is closed as the work progresses by boarding, but a small space is left at the top through which the progress of the operation can be observed, and fresh additions of material made as required to allow for sinking of the tan, &c. ,The quantity of lead used varies considerably, or from about 3 tons to 7 tons in a layer of materials, so that in a large stack there may be something like 85 tons of lead. ,In stacks of very large area it is usual to construct chimneys throughout the mass, whereby the steam which is produced during the operation is carried off; in stacks of small area, these chimneys are not required, as the space around the side walls of the shed affords a sufficient outlet. ,When the stack is built up it is left for a period of about three months. During this period the stack gets quite hot (140 F.) through the fermentation of the tan which sets in; large quantities of carbonic acid gas are given off, and the acetic acid is converted into vapour. The "blue lead" is gradually converted into "white lead." At the end of three months the stack is pulled to pieces. As the boards are removed it is found that the lead which has been corroded still retains the form of the blue lead, but is more bulky in volume, is white or greyish in tint, and opaque. The corrosions are not of a uniform character throughout the whole of the stack ; in some places they are porcellaneous and flaky, are firm to handle, do not break up, and give the best quality of white lead ; in other ,parts of the stack the corrosions are soft, easily crumble to a fine powder or dust when handled, and do not give a good quality of white lead. In some places the lead may be discoloured owing to a variety of causes, such as the presence of tarry matter in the acid (especially when crude pyroligneous acid is used), by droppings of coloured water from the layer of tan on to the lead, &c. In chemical composition the corrosions will vary ; in some places they will approximate closely to the normal composition of white lead, 2 Pb C O 3 , Pb H 2 O 2 , in others more nearly to that of 3 Pb C O 3 , Pb H 2 Og, while in others they consist of the normal carbonate, Pb C O 3 .