The construction of a stack is a very simple and rude operation. Layers of dung or tan, or a mixture of the two, are so arranged as to imbed a large number of earthenware pots, each containing some acetic acid. These pots are about 4 or 5 inches in diameter, and about 7 or 8 inches high; a coil of lead is placed in each pot, and buckles or gratings of lead supported on oaken bearers are laid across and on top of the pots; boards are laid to cover the whole, and form a floor. The stack is composed of a number of such layers of pots, bearers, and buckles or gratings, raised one upon another. A stack chamber is a brick enclosure 10 or 12 feet square, and 20 or 25 feet high; such a chamber will contain about 70 tons of lead when stacked and piled. In a white lead factory several of these chambers are built side by side, and when they are in full operation a set of chambers will contain as much as 700 or 1000 tons of lead. Only the purest kind of lead will be suitable for conversion in this stack process of making white lead, the common varieties being inadmissible. Messrs. Pontifex and Wood have furnished the following analyses of lead used for white lead making. Copper. Antimony. Iron. Spelter. Silver. Lead. A 0·00700 0·00490 0·00200 0·00080 0·00100 99·98430 B 0·07580 0·00320 0·00220 0·00320 0·00200 99·91360 C 0·00340 0·00460 0·00120 0·00070 0·00350 99·98660 D 0·05260 0·00740 0·00150 0·00180 0·00400 99·93270 E 0·00940 0·00210 0·00160 0·00100 0·00075 99·98515 F 0·02360 0·00580 0·00210 0·00180 0·00100 99·96570 The presence of silver, copper and iron in the lead would damage the colour of the white lead resulting, and other admixtures retard or prevent the progress of conversion. In olden times horse dung was the only imbedding material used in the stack arrangement. This material when heated evolves gases which seriously interfere with the colour of the resulting corrosion. Dung has been almost superseded in this country by tanners’ refuse; in Belgium dung is yet employed, and in some places a mixture of dung and tan. Where dung is used, the process of corrosion of the lead goes on more quickly than when tan alone is employed, but the use of tan offers great advantages, especially this one: that it does not give off gases that damage the white lead. The operations of charging and discharging these chambers are principally the work of women, and are most laborious and fatiguing. In emptying the chambers and stripping the stacks, the women are fully exposed to the heated gases which are yielded by the decomposing tan, and the heated and corroded lead. These gases, in themselves most injurious to health, are not to be compared in this respect to the dust which pervades the air and fills the chamber in which these women work.