finally the roof. The portion collected in the first compartment is frequently contaminated with unburnt oil, which has become volatilised by the heat of the furnace, and condensed again when it reaches a cold place. This oily lamp-black should always be collected separately, as the oil it contains often causes spontaneous combustion in the lamp-black.

A different method of constructing the chamber is used in some works. In this case the partitions do not run from side to side, but from bottom to top, large openings being left alternately in the top of one partition and the bottom of the next. The chamber is of about the same size as the last described, and has a similar arrangement for burning the oil. This form of plant (shown in Fig. 25) necessitates a door being provided for every compartment, whereas in the last form of plant one or, at the most two, doors only are required. In some cases a large room is provided at the end of the chamber into which the gases, &c., pass before reaching the chimney; here the last particles of black condense, so that gaseous matter only passes into the chimney.

Fig. 25.-Lamp-black chambers.

The amount of lamp-black obtained by any of the processes just described varies very much; it depends upon the kind of oil or grease used, as well as on the completeness of the combustion and of the condensation, the effect of which on the yield must be obvious. Approximately the yield may be taken as 25 to 30 per cent. of the weight of the oil used. The relative proportions of lamp and vegetable blacks produced vary considerably.

3rd Method. The arrangements described above are those in general use, but are capable of very great improvement. A lamp-black furnace of an improved construction was patented in 1879 by Messrs. Winslow, Humphrey & Buttrick. It is shown in Fig. 26. A number of these furnaces are usually arranged

in a row in front of condensing chambers, such as are used in the older methods. From the front wall, a, of the condensing chamber projects an iron plate, f, which is supported on suitable brickwork; this forms the floor of the fire-chamber, e; the pot, o, is kept at a red heat, and receives the oil or other suitable

material in drops from the pipe, lk, connected with the main supply pipe, m; this oil burns with a more or less smoky flame, and the products pass through the opening, h, into the condensing chamber. A quantity of the oil will drop on to the iron plate and enter into combustion; this serves to keep the plate and the pot, o, red hot. To prevent the temperature getting too high, a considerable air-space is left under the plate, while the openings, g g, admit, and allow the circulation of, air. i is a door by means of which the admission of air for the combustion of the oil is regulated. A comparatively low temperature is required and should be just above the burning point of the oil; this, however, varies, some oils burning below 500° F. and others above 600° F.

The whole arrangement being outside the condensing chamber is readily accessible for the purpose of regulating the amount of oil, the temperature of the furnace, and other details affecting the production of the black.

4th Method.-A somewhat different method was patented in 1880 by Nawrocki. The oily matters used were burnt in a series of lamps arranged in a row, the oils being heated, if necessary, to ensure proper combustion. Above the burners was arranged an iron plate, against the bottom of which the

flames from the burners impinged; consequently, the combustion was rendered incomplete. This plate is circular in form and made to revolve; it is kept as cool as possible by the upper surface being divided by ribs into a number of compartments through which a constant stream of cold water is made to flow. The action of the apparatus is as follows:-The lamps are lighted and the plate caused to revolve; the black forms on the plate where it is in contact with the oil flames, while the revolution of the plate continually brings a fresh surface for the deposition of the black; as the plate revolves it comes into contact with scrapers, which scrape off the black into suitable receptacles, so that a fresh, clean, and cold surface of the iron plate is always exposed to the flames, and the maximum amount of black is produced by the combustion of the oil.

The process is a slow one, and the yield of oil-black is not so great as in the processes above described. The black obtained in this way is rather more granular in its formation, and has a tendency to be somewhat harder and greyer in tint. Should, by any accident, any of the black be exposed to the action of the flames for too long a period, it is liable to be overburnt, and is thereby rendered hard and almost unusable as a pigment.

5th Method. In America a large quantity of black is produced by the combustion of the natural gas which flows out of the ground in many of the oil regions. This black is made in large quantities, and sold under the names of gas-black or carbon-black. The principle on which its manufacture is based is the same as that of the last method-viz., the cooling of the flames of the burning gas by iron plates. The form of these plates has altered during the time which has elapsed since the industry was introduced; at first stationary plates, with long trough-shaped upper surfaces, were adopted, and were kept cool by means of a current of water; these were found to be subject to certain defects. Thus, water was condensed on the lower surface, which interfered with the proper condensation of the black, and made the latter damp; then, the black formed, not being immediately removed from the action of the flames, was burnt, and became granular in form, greyer in tint, and much harder. For the purpose of removing the black from the plates, scrapers were passed over the depositing surface from time to time. Of late, these fixed depositing plates have been replaced by revolving plates or cylinders which, during the time the black is being deposited, automatically revolve, and so the black as it is formed is removed from the action of the flames, and, therefore, cannot be overburnt. As the plates or cylinders

revolve, they come into contact with fixed scrapers, which remove the black from the surface as fast as it is formed. Carbon-black differs from oil-lamp blacks in being granular in form and rather denser; it is blacker in hue than any of the other black pigments. For grinding this black, steel mills are preferable to stone mills. It is the purest form of carbon-black, made quite free from any trace of the unburnt oil often present in lamp-blacks, and from any trace of mineral matter. (See Oil and Colourman's Journal, June, 1891; Journ. Soc. Chem. Ind., February, 1894.)

PROPERTIES AND COMPOSITION OF LAMPBLACKS.-Lamp-black is in the form of a black, flocculent powder with a fine texture; in hue it is, usually, what is termed a jet-black, although some samples have a faint brownish tinge. It has great colouring and covering powers. It is rather difficult to mix with various vehicles, especially with water, but, when mixed, it works well as a paint; it dries rather badly when used as an oil paint, especially those samples which contain unburnt oil. It is perfectly permanent as a pigment. It is sold in the form of powder and also as a paste ground with oil, of which it takes 27 per cent. ; or with turps, of which it takes 55 per cent. to form a stiff paste. Vegetable black is a more voluminous black than lamp-black, of a deep jet-black colour and very fine texture. Its colouring and covering powers are rather greater than those of lamp-black. Carbon-black has similar properties to vegetable black, but has a slightly more granular structure. These blacks are used for making printing inks, varnishes, paint, and for other purposes where a fine black is required.

Lamp-black consists almost entirely of carbon, but there is a small quantity of moisture and mineral matter present in all samples. Vegetable and carbon blacks are nearly pure carbon. The following analyses of lamp and vegetable blacks will show their average composition :—

Samples of lamp-blacks are sometimes met with containing small quantities of oil; these come from the first condensers; such lamp-black is defective, for two reasons-1st, the oil prevents the paint made from the black drying properly; 2nd, the oil is liable to cause spontaneous combustion, so that it is no uncommon occurrence to find a cask of lamp-black almost red hot after standing some time.

Carbon-Blacks.-The carbon- or gas-blacks made in America are of very pure quality, three samples examined by the author had the following composition:

These blacks have small quantities (2 to 3 per cent.) of gas occluded within them, these have not been separately investigated in the above analyses.

ASSAY AND ANALYSIS OF LAMP-BLACKS.-The blacks just dealt with can be assayed for colour, colouring power, and covering power by the usual methods. Lamp-blacks should contain but a small quantity of mineral matter, not exceeding about 3.5 to 4 per cent. To determine the amount, 2 grms. should be weighed into a platinum crucible, and the black heated over a Bunsen burner until all the carbon is burnt off, and nothing but a greyish ash remains; the crucible and its contents are now weighed, and the weight of the ash ascertained. Water can be determined in the usual way. The difference between the amount of water and ash and 100 may be taken as carbon. If the black shows signs of its containing oil, the amount of this impurity may be ascertained by treating a known weight, say about 5 grms., with petroleum ether in a Soxhlett extraction apparatus. The petroleum ether will take out the oil; the ethereal solution is run into a weighed glass, and the ether evaporated off in the water bath; the combined weight of the glass and oil is taken, and the amount of oil ascertained. Vegetable blacks are analysed in the same way; they should have no oil, and should not contain more than 0.5 per cent. of water, or 0.25 per cent. of ash. Carbon-blacks should resemble vegetable blacks in their composition.

BONE-BLACK.

Bone-black, or animal black as it is often called, is prepared from bones by a process of charring them in a closed vessel; the organic matter they contain is decomposed, much volatile matter is given off, and carbon is left behind along with the mineral matter of the bones.

The process of making bone-black may be carried on under two conditions-1st, the volatile products are not collected; 2nd, they are collected. The first plan is the one generally adopted, as there is very little use for the oily matter, known as Dippel's or bone oil, which is obtained by the dry distillation of bones.

1st Process. When the volatile products from the charring