To be decomposed the hydrocarbons of the petroleum or their vapors must come in contact with the hot sides of the distilling apparatus. Hence the latter has to be so arranged that the heavy portions and their vapors come in contact for as long a time as possible with a large portion of the overheated surface. The hotter the sides of the still are, the more rapidly decomposition proceeds and the more light and correspondingly heavy parts are formed. Hence it is of great importance to be able to regulate the temperature so that distillates suitable for illuminating purposes are obtained with the cracking stills to be described later on.

Although the nature of the decomposition the heavy oils undergo by this operation is not yet thoroughly explained, it may be supposed to be based upon dissociation and polymerism. The distillates obtained by cracking have unfortunately not yet been subjected to examination, but it would seem to be incorrect to consider them as consisting of the same hydrocarbons as those obtained from crude petroleum, they being very likely richer in aromatic and non-saturated hydrocarbons. The cracking process based upon decomposition with the assistance of hot iron may be explained by experiments of Letny, Ragosin and others. They conducted petroleum residuum through iron pipes heated to a slight red heat and obtained distillates with 15 to 20 per cent. of benzol and its homologue (anthracene). A similar process must take place in cracking heavy oils. The cracking still heated to a gentle red heat may be considered a large pipe in which the heavy oils suffer decomposition by coming in contact with it, and form, though not to the same extent as in the abovementioned experiment, a considerable percentage of aromatic hydrocarbons. That the hydrocarbons formed by the cracking process have a different chemical composition from those occurring in crude oil is also proved by their behavior when subjected to treatment with sulphuric acid and lye. The consumption of acid and lye is incomparably larger, the action of the chemicals far more intimate, and the refined product more

changeable and inferior in quality notwithstanding the most careful treatment. Petroleum prepared from such oil has a stronger odor, becomes dark very rapidly and possesses but little burning power. Nevertheless the cracking process is an indispensable part of the manufacture of petroleum, because it offers the means of realizing in a lucrative manner from residuum otherwise difficult to utilize, and of increasing the yield of petroleum. However, the process is not cheap, and its employment is rendered possible only by the small value of the crude material. In working residuum by the cracking process the consumption of fuel is very large; thus, for instance, while for the distillation of illuminating oil from crude oil about 22 pounds of coal are used for 220 pounds of crude oil, for the same quantity of residuum about 50 per cent. of coal is required, and it must further be considered that the product of distillation obtained cannot be utilized as such, but has to be subjected to further rectification, whereby residuum is formed that must again be cracked. The process of cracking also causes losses by the unavoidable formation of permanent gases (about 6 to 8 per cent.) which in well arranged refineries are caught and used for illuminating and firing purposes, but as a rule are allowed to escape. The separation of the heaviest hydrocarbons, asphalt and finally coke as the principal quantity, must also be taken into consideration; it generally amounts to from 10 to 25 per cent. or more of the residuum worked.

The cracking process is carried on in stills especially constructed for the purpose. The upright form of still is most suitable because the oil vapors, having to traverse a long distance in the still itself, have a chance to condense partially and in flowing back on the overheated sides to suffer decomposition and vaporization. The still should be provided with a large condensing dome and with suitable dephlegmators, the use of the latter being absolutely necessary to increase the yield. They should be so arranged that the oil-vapors meet large condensing surfaces, that the heavy liquid oils can flow back into

the still, and that only the most volatile oils are caught in the distillate-distributor. The still is best provided with a convex bottom; a flat or inward bulging bottom is not suitable, because the first, if of wrought iron, burns through very readily, and if of cast-iron cracks easily, and besides the superheating of the oil and the separation of the coke are more difficult to effect. The same objections apply to the inward bulging bottom, to which must be added the filling space lost by its shape. The still is generally constructed of cast-iron, the temperature of the sides of the still being with this material more constant and the durability of the still itself greater. Where for economical reasons the still has to be constructed of wrought-iron, the most exposed part of it, the bottom, should be of cast-iron. The use of a wrought-iron bottom is objectionable, because it loses its shape and is destroyed in a short time by the high temperatures prevailing in the still and the separation of coke. The only advantage of a wrought-iron bottom is that the still can be more rapidly heated. Wroughtiron stills with bottoms which have been made especially resistant by the addition of ferro-manganese have recently been introduced, but whether successfully or not, is not yet known. Figs. 110a and 110b show the construction and bricking-in of a cast-iron still. It consists of an upright cylindrical body and a hemisphere of cast-iron. Only very small stills are cast in one piece; larger stills are cast in two parts and joined by bolts and nuts, or during casting the cylinder is cast upon the hot hemisphere. As seen in the illustration, the hemisphere is provided with a shoulder a by means of which the still rests upon the brick work. On the hemisphere is the beak S which projects from the brick work; it is provided with the valve Vand serves for the discharge of the hot asphalt. On the lid of the still are a conduit for filling the still, a conduit for direct steam, and finally the condensing pipes cc. The lid is screwed to the flange of the still. The man-hole D serves for removing the coke and cleaning the still. The arrangement for firing is as follows: The flame from the fire-place A passes the fire-bridge

B, ascends at N through the arch, washes the bottom of the still, ascends laterally, and through the flue Z, passes round the still. When the slide T, is closed, the flame ascends laterally, again washes the still at Z2, superheats the sides, and on opening the slide T passes into the smoke-flue. If the work requires it, the slide 7, is opened and the slide T closed; the heating gases then wash the still only once and pass immediately into the smoke-flue. With this mode of operation the still is filled with residuum or the oil to be decomposed. A gentle fire is then started to evaporate any water which may have been carried along. When the more volatile oils have been driven off the actual cracking process commences, whereby the fire must be kept in accordance with the products of condensation, i. e., it must be decreased or increased according to whether heavier or lighter products are formed. The distillates when thoroughly dephlegmated are condensed in the manner previously described. However, by this distillation paraffine or paraffine-like products are formed which readily clog the condensing pipes, and hence the cooling water must be kept warm and frequently boiling hot.

Regarding the capacity of the stills no general rule can be laid down; they should, however, be of sufficient capacity to hold all the crude oil residuum. To save room and to avoid repeated firing and heating, the still may once or twice be refilled to the original depth towards the end of distillation. Further refilling is, however, inadmissible, because the products obtained are not light enough and the quantities of coke separated so large, that the operation and the still would positively suffer. The height of the still is generally 2 to 21⁄2 times greater than its diameter. The thickness of the sides varies. While according to some the sides should be cast as thin as possible and the bottom portion thicker, there are many stills with sides from 2.36 to 3.15 inches thick, and bottoms up to 5.51 inches thick. Both of these extremes should be avoided, because, in the first case, the stills are not durable, and in the latter, the consumption of iron used in their construction, and