of fuel for heating them, is unnecessarily large. It is recommended to cast the stills with bottoms 2.36 to 3.15 inches thick (and if of specially tough cast iron only 1.57 inches thick), and Fig 111a.

with walls 1.57 to 2.36 inches thick. Such stills answer all demands and show no cracks or other defects even after continuous use for a long time.

Figs. 111a and 111b show a form of a still K much used for

working readily decomposable residuum. It consists of a somewhat lower cylindrical upper portion and a hemispherical bottom to which is cast the discharge aperture S. The latter projects from the brick work and is closed by a lid screwed on. All other arrangements are similar to those of the previously described still.

The mode of firing is, however different, the heating gases coming in contact with a far greater surface of still. The flame

passes from the fire place A through the arch N, washes the bottom of the still and, if necessary, passes through the flue Z, into the smoke-flue. If the slide 7 be closed and T, opened, the flame passes into the flue Z, and into the smoke-flue only after washing the sides of the still.

From the dimensions of the flues it will be seen that much larger surfaces are swept by the heating gases and that the temperature of the latter is much higher...

The mode of operating is nearly the same as with the still previously described, but in most cases the work is carried on

to complete coking, to avoid as much as possible the opening and closing of the man-hole D.

The essential portion of the cracking process is the production of specifically lighter oils, the formation of which is effected, as previously mentioned, by overheating the heavy oils on the sides of the still, which presupposes their vaporization. A portion of the heavy oils is unavoidably carried along with the more

volatile oils, whereby the yield of the latter and their quality are influenced. More intensely acting dephlegmation is therefore absolutely necessary, so that by as large a condensing surface as possible of the dephlegmator the specifically heavy oils are completely condensed and can flow back into the still, where they again suffer decomposition on the overheated sides.

Fig. 112 shows the arrangement of a dephlegmator which has been used with great success. The oil vapors enter the apparatus at a, are partially condensed and flow back into the

still at b, while the non-condensable vapors ascend at c into the space inside the jacket m, where they are also condensed, the condensed products also running back into the still at e and

only the lightest portion flowing at d into the condensing apparatus. When towards the end distillation has proceeded so far that no oils appear at d, the distillate is discharged by simply turning the cock at f.

There are a number of other condensing drums, which, however, need not be deseribed.

Figs. 113a and 113b show a form of still which may be recommended, it consisting of a wrought iron upper portion with

cast-iron hemispherical bottom. Such a combination is advisable for several reasons. A still of the same capacity constructed entirely of cast iron is more expensive and less durable. In case the bottom from one cause or another becomes defective it can be readily removed and replaced by a new one. Besides, the wrought iron portion is much more rapidly overheated than is the case with cast iron.