is anthracite to common coal. Certain kinds of coal, as those of Lamure and Macot in the table, are classed with the anthracites, on account of the similarity of their properties; these, however, are not the result of an advanced stage of the process of decay, but of the agency of heat accompanying the later elevations of primary rocks. Anthracite is eminently homogeneous and without impressions of plants; it is black, has a decidedly vitreous lustre, a powerful play of colours and a conchoidal, sharp-edged fracture. Its structure is massive. The amount of ash, composition and specific gravity of those specimens which have been examined, are collected in the following table :

Regnault found 0.37, Jacquelin 0.58 to 2.85 nitrogen. The ash consists of silica, alumina, oxide of iron, and according to more recent observations, also contains chlorides, which, volatilizing during combustion, damage the metallic portions of the stove or grate. Professor Johnson gives the following composition for the ash of some American anthracites:

A careful examination of the composition of fossil fuel, as shown in the foregoing tables, clearly exhibit a gradual increase in carbon as compared with woody fibre, until at length the hydrogen and oxygen do not amount to 10 per cent. This circumstance becomes still more obvious, when the amount of ash is deducted, as in the following table:

In fact, all three ingredients have disappeared in certain proportions, whilst the carbon, which is always the preponderating element, is least affected.

EFFECT OF HEAT UPON FUEL.

As combinations of organic origin, the kinds of fuel which have here been described, are not very simple in constitution, and therefore offer but slight resistance to external modifying causes; they are bodies easily decomposed, and like all other chemical combinations can only exist within certain limits of temperature, the limit being greater for simple than for complex bodies. The varieties of fuel are not volatile; the chemical equilibrium amongst their elements is destroyed by an increase of temperature long before volatilization can take place; and the decomposition caused by heat, is simply an overthrow of the existing arrangement of the elements, while an immediate re-arrangement ensues with the formation of new compounds capable of existing at the higher temperature. The nature of the new products is therefore mainly dependant upon the latter, and these must vary in quantity still more than in quality as the temperature of decomposition is raised or depressed; while the admission or exclusion of air (oxygen) during the process will still further modify the result. In the former case, the products are immediately subjected to the energetic chemical action of oxygen, with which their elements are forced to combine, and combustion results as a secondary process. In the latter, where decomposition by heat is effected without access of air, we have what is called the process of dry distillation. The products of this operation can be conveniently collected and studied, and the process demands par

ticular attention, not only because it obtains in every instance of combustion, but because it is instrumental in producing an important transformation of certain species of fuel. It would, indeed, be a very erroneous idea to suppose that the combustion of wood, coal, or other species of fuel, was simply the result of a direct union of atmospheric oxygen with their elements; on the contrary, the heat produced by the burning of one portion causes the dry distillation of the internal parts nearest to it, before they are bought into contact with the air. When these have at length become exposed, they are then first acted upon by the oxygen. In short, it is not the wood which we see burning when a billet is ignited, but the products of its decomposition by the agency of heat. The main points in this process of decomposition by heat in closed vessels (dry distillation) are the following. From the moment in which the elements are forced by the heat to abandon their former state of equilibrium, three circumstances concur in regulating the formation of the new products. These are: first, the temperature; second, the degree of chemical affinity among the elements, increased by their being in the nascent state; and third, their volatility. Hydrogen and oxygen possess the latter property in an eminent degree, while carbon is not in the least volatile; there is a tendency, therefore, in the former to separate and pass off in the form of gas; but chemical affinity coming into play obliges them to unite, and form new compounds, partly with each other, partly together or separately, with carbon. Among the combinations that are possible, those of course will result in which the elements have the strongest affinity for each other under the circumstances and existing temperature. Hydrogen and oxygen combine in the simplest and most stable manner to form water; the excess of hydrogen which is common to all fuel, takes up as much carbon as the temperature admits of, forming light carburetted hydrogen and olefiant gas, while at the same time the united action of the two other elements upon the carbon, gives rise to a series of ternary compounds. The simultaneous production of all these bodies gifted with powerful chemical affinities at a high temperature, induces fresh activity, and products of a subsequent action are the final result. In short, the nature of the process admits of the production of an almost innumerable series of bodies, as many perhaps as there are mathematical combinations, binary and ternary, for every degree of temperature. Many of these products are formed in

every case of dry distillation, some of which require more particular notice. A liquid is obtained in addition to the gases (carbonic acid, carbonic oxide, hydrogen and light carburetted hydrogen), the lower stratum of which is an aqueous solution of substances, amongst which acetic acid and ammoniacal salts are the most prominent; the upper stratum being a mixture of bodies analogous to the resins and ethereal oils, and very rich in hydrogen, is technically called tar. Pyroxylic or wood spirit, a kind of alcoholic compound, is obtained when wood is the substance charred. The products vary when any of the varieties of coal are submitted to dry distillation, we then obtain from the tar, in addition to the substances discovered by Reichenbach (paraffin, picamar, kreosot, kapnomor, pitacall, and naphthaline), pyrogeneous resin, and numerous oils and coal-tar-naphtha, a liquid containing various neutral, basic, and acid bodies.

The less oxygen there is in fuel, and the more the hydrogen preponderates, as is the case in coal, the more numerous are the products of decomposition, which this element forms with the carbon. However much the formation of highly carbonaceous products may be facilitated by a suitable temperature, in no case are we able with wood, and still less with turf, brown or common coal, to compel the two other elements to combine with and eliminate the whole of the carbon; a certain portion of fixed carbon is always left, its quantity depending chiefly upon the degree of heat applied. The original form and structure of wood, brown coal and turf are retained by the charcoal left by each, so that year-rings and cells may be distinguished in wood-charcoal, which discover the kind of wood from which it was produced. Coal is affected differently, having a different elementary composition. Some kinds pass during the process of decomposition into a soft state, or kind of fusion, so that the gaseous products of decomposition, are evolved in bubbles, as it were, from a paste. The carbon left by common coal is called coke; it is filled with cavities, is more or less dense, and has in general no resemblance whatever to the form of the original coal. The natural moisture, as well as the oxygen present in the fuel, which during combustion produces water with the hydrogen, frequently prevent the attainment of the very high temperatures required in furnaces, locomotive engines, and for other purposes. For this reason, it has been the practice, from a very early period, to make use of dry distillation as a means of removing those constituents

of the fuel which absorb heat, or as a means of concentrating the heating power, and confining it to a smaller space. This is the object of charring wood, or of converting it into charcoal, which series of operations have since been applied to peat, brown coal, and particularly to coal itself, in which latter case the process is called coking. From the series of natural, a series of artificial fuels are thus obtained, the production of which we proceed to describe. The manufacture of charcoal and coke is in itself a distinct operation, not directly connected with those to be mentioned hereafter, in which the dry distillation of certain kinds of fuel is practised for obtaining tar and the gases simultaneously evolved.

WOOD CHARCOAL.

On the production of Charcoal.-On examining minutely the process of combustion, when, for instance, the lower end of a chip of wood is ignited, two well-defined periods will be observed. At first, a bright flame caused by the ignition of the volatile products of decomposition, which grows less intense by degrees, and is at last extinguished when the gases cease to be evolved, and the process closes with the faint glimmering of the remaining charcoal. If the chip is gradually inserted into a narrow closed glass tube as the flame goes out, the charcoal cools without glimmering from want of air. It is even possible completely to char the chip, in the manner mentioned above, when the access of air is prevented from the beginning by heating the wood in a close vessel. The original mode of preparing charcoal on a large scale was conducted upon the former principle, without entirely excluding the latter; but in the more recent methods large close vessels have been resorted to. Whatever plan is adopted, the amount of charcoal is always found to be greatest when time is allowed for the oxygen to combine with the hydrogen of the wood and form water. Experience has in fact proved that the slow process of charring is decidedly preferable; this may be seen from Karsten's experiments, by the side of which we place those of Stolze and Winkler. The following are the quantities obtained from 100 parts of air-dried wood: