When the carbon, oils, resin, and water are associated together in certain proportions, they constitute tar. Soft pitch is produced if the tar be so far heated that the water is expelled; and hard pitch (resin blackened by carbon) when the oils are volatilized.
In all cases of ordinary combustion, carbonic acid is formed by the red-hot cinders, or by gases or other compounds containing carbon, acting on the oxygen of the air. This carbonic acid is discharged in general as an invisible gas. If the carbonic acid pass through red-hot cinders, or any carbonaceous smoke at a high temperature, it loses one particle of oxygen, and becomes carbonic oxide gas. The lost oxygen, uniting with carbon, forms an additional amount of carbonic oxide gas, which passes to the external atmosphere as an invisible gas, unless kindled in its progress, or at the top of the chimney, when its temperature is sufficiently elevated by the action of air. Carbonic oxide gas burns with a blue flame, and produces carbonic acid gas.
Black smoke is always associated with carburetted hydrogen gases. These may be mechanically blended with the oils and resins, but must be carefully distinguished from them. They form more essentially, when in a state of combustion, the inflammable matters that constitute flame.
2. Smoke from Charcoal, Coke, and Anthracite, is always invisible if the material be dry. A flame may appear, however, if carbonic oxide be formed.
3. Wood or Pyroligneous Smoke is rarely black. Water and carbonic acid are the products of the full combustion of wood, omitting the consideration of the ash that remains.
4. Sulphurous Smokes. Tons of sulphur are annually evolved in various conditions from copper-works. Offensive sulphurous smokes are often evolved from various chemical works, as gas-works, acid-works, &c.
5. Hydrochloric Acid Smoke is evolved in general in large quantities from alkali works.
6. Metallic Smokes—when ores of lead, copper, arsenic, &c., are used—often contain offensive matter in a minute state of division, and suspended in the smoke evolved from the furnaces.
7. Putrescent Smokes, loaded with the products of decayed animal and vegetable matter, are evolved at times from drains in visible vapours, more especially in damp weather. The fœtid particles, when associated with moisture in this smoke, are entirely decomposed when subjected to heat.
Dr. Ure says, speaking of the cause of the ordinary black smoke above described, “The inevitable conversion of atmospheric air into carbonic acid has been hitherto the radical defect of almost all furnaces. The consequence is, that this gaseous matter is mixed with an atmosphere containing far too little oxygen, and instead of burning the carbon and hydrogen, which constitute the coal gases, the carbon is deposited partly in a pulverized form, constituting smoke or soot, and a great deal of the carbon gets half-burnt, and forms what is well known under the name of carbonic oxide, which is half-burnt charcoal.”