For working on a very large scale, Heyl's extracting apparatus, shown in Fig. 14, is very suitable. It consists of a battery of four or more cast iron or sheet iron cylinders, _A_{1} to _A_{4}, communicating with each other and surrounded by steam jackets. The extracting vessels are so arranged that they can be emptied by tilting, which is rather inconvenient, as all the pipes have to be unscrewed. In each cylinder close above the bottom is a perforated plate covered with fine wire-gauze, upon which the material to be extracted is placed. The cylinder is filled to the top, and, after placing a similar plate upon it, the upper opening is closed by a lid suspended to a crane. The cylinder, as well as the lid, is provided with a broad flange, between which is placed a hemp tissue firmly pressed together by 12 clamps to serve for packing. After filling the cylinders with the material to be extracted and arranging the packing, the solvent (bisulphide of carbon) is conducted from a reservoir through the principal pipe, _B_, to the extracting vessels, and is introduced into _A_{2} by opening the cock _C_{2}, which communicates with the pipe _B_. The bisulphide of carbon passes through the bent pipe _D_{1}, enters through the cock _E_{2}, below the false bottom of the cylinder _A_{2}, and, after penetrating the mass and filling the cylinder, runs through the cock _C_{2} of the bent pipe _D_{2}, and the cock _E_{3} into the cylinder _A_{3}, reaching the fourth cylinder in the same manner through the cock _C_{3}, the pipe _D_{3}, and the cock _E_{4}. From the last cylinder it passes as a thoroughly saturated oil solution into a reservoir, in which a vacuum has been created to promote the circulation of the fluid in the entire apparatus. After a quantity of oil solution corresponding to the contents of the cylinder _A_{4} has arrived, the cock _G_{4} is closed and the cock _C_{4} opened, whereby the cylinder _A_{4} is connected with _A_{1} by the bent pipe _D_{4} and the cock _E_{1}. After the exhaustion of the contents of the cylinder _A_{2}, which is recognized by means of the glass tube _H_{2} placed on _D_{2} by the fluid running off being colorless, the cocks _C_{1} and _E_{2} are closed, and _C_{2} and _E_{3} opened, whereby the solvent runs into _A_{3}, and from there to _A_{4} and _A_{1}; _A_{2} being omitted. To effect this omission, and at the same time not to prevent the introduction of bisulphide of carbon, _C_{1}, _C_{2}, _C_{3}, and _C_{4}, are so-called two-way cocks, which, when placed in one position, connect the principal pipe _B_ with the branch pipes _D_, but interrupt a further flow through the principal pipe _B_; while in the other position they close the pipes _D_ and open the principal pipe _B_.