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constantly keeping the sand-bath at the same degree of temperature. It was in the rapid carbonization that the results differed most from each other, because, in this case, it is still more difficult to regulate the temperature. The quantity of charcoal obtained by means of the rapid carbonization varies, for 100 parts of the matter employed, between 11.90 (the produce of old oak), and 16 39 (that of young oak); but, in the slow carbonization, the quantity of charcoal obtained is nearly double, or at the least one-half more. It varies from 24.20 (the produce of lime-wood), to 27.50 (the produce of young Norwegian spruce). In both modes of carbonization, the quantity of ashes remains the same: it varies, in general, from 2.75 (the produce of fern) to 0.11 (the produce of old oak wood); but, in most cases, it is below 0.4.
Like unaltered vegetable fibre, fossil wood, on being carbonised, retains its external form completely, and only undergoes a diminution of size. This preservation of the external form after carbonization, that is to say, after a complete decomposition, is a phenomenon without example in inorganic nature, and one exclusively peculiar to unaltered vegetable fibre, fossil wood, brown coal, and some sorts of black coal. Other kinds of coal, in the process of decomposition by an ardent heat, lose more or less their form; and, by the difference which they thus exhibit, they already afford an indication beforehand of what their composition must be.
It may be without rashness asserted, that fossil wood and lignite, or brown coal, are still at the present day, so to speak, in a train of developement. This is proved by the frequent occurrence, in brown coal mines, of pieces of combustible, which present an evident transition from fossil wood to brown coal,one extremity of the specimen being fossil wood, the other brown coal. With regard to black coal, there is not equal reason for supposing that the formation of that combustible is still going on, or that a change of relation in its elements still continues to be effected, although this is not improbable.
From the frequent variations which fossil wood presents in its passages into brown coal or lignite, it might already be expected not to afford, as the residuum of its carbonization, a constant quantity of charcoal, as was seen to be the case with regard to
unaltered vegetable fibre. According as fossil wood approaches more or less to the nature of brown coal, it furnishes a greater or less quantity of charcoal; but, in the carbonization of fossil wood, as well as in that of brown coal, the quantity and kind of the products formed depend upon the degree of the temperature, although, in the species which come nearest to brown coal, the limits are already much more restricted. In general, fossil wood, submitted to distillation in the dry way, affords the same quantities of gas as the fibre of unaltered wood; but it yields less water, and still less of that oil, of a peculiar and disagreeable smell, by which all the brown coals are instantly recognised. The empyreumatic acid is then only formed in very small quantity; but, on the other hand, the formation of alcohol is much more considerable than in the case of unaltered vegetable fibre. Those lignites or brown coals, which, from their external characters, visibly present a passage into black coal, afford in the dry distillation water, with a very small quantity of fetid oil, and often, furnish so much as 70 per cent. of pure charcoal.
Thus, therefore, says M. Karsten, those brown coals, the common Braunkohle of Werner, from which the Moorkohle of the same mineralogist does not differ, in distillation surpass a great many black coals, as to the quantity of charcoal obtained from them. Add to this, that the specific gravity of these brown coals rises to 1.2881, and is consequently higher than that of several varieties of black coal, which cannot be attributed to the quantity of earthy matter and oxide of iron, since these brown coals frequently do not contain one per cent. of them.
The quantity of ashes afforded by fossil wood and brown coal is very variable. In the species submitted to examination by M. Karsten, it varies from three-fourths to more than fifty per cent., which latter is the case with earthy brown coal. This produces a serious inconvenience in the employment of these combustibles; for the ashes, by resting upon the substance which is burning, oppose combustion to such a gree, that a stronger current of air must be employe the proper nature of the combustible, without this circu would require. Hence the great difficulty of employ substance advantageously for the purpose in view. of fossil wood and brown coal contain no traces of f Silica, alumina, oxide of iron, sulphate of lime, a
and magnesia, are the substances which are found in the residua of the combustion of fossil wood and brown coal. They present themselves in very different and very variable proportions, which depend upon the local circumstances under the influence of which the deposition of matter has been effected in the natural beds of these combustibles.
In black coal, the quantity of charcoal which may be obtained, by means of distillation in the dry way, varies still more than in the different sorts of brown coal, comprising also fossil wood. M. Karsten has not met with any black coal, which, on being distilled, has furnished less than 48 per cent. of charcoal. From this number, the quantity of residuum in charcoal rises to 90 per cent. Between these two limits there is scarcely a number to be found that would not answer for the produce in charcoal, or coke, of some kind of coal. Striking differences, however, are remarked in the external form of the carbonized coals called cokes.
In some the form of the coal remains unchanged, the volume only being diminished, as in charcoal from fresh vegetable fibre, fossil wood, and brown coal. Others remain unchanged in form and volume, while some swell and expand more or less. In order to observe correctly these different relations, it is necessary to use the coal we intend submitting to dry distillation in the state of powder. Coal of the first kind affords a coke in a dusty pulverulent state, without the least cohesion, just as in brown coal. In coal of the second kind, the powder is conglutinated into a cake, often very solid and tough, but without any swelling or intumescence. The fine powder, in coal of the third kind, melts, and forms a homogeneous mass, which takes the form of the retort in which it is distilled, and frequently swells so much as to choke up the retort.
Here the author divides coals into three classes, which he establishes from the external appearance of the charcoals or cokes which are produced by them. For the object which he -proposes to himself, M. Karsten distinguishes,
1st, The coals with pulverulent coke, (Sand Kohlen) ; 2dly, Those with conglutinated coke, (Sinter Kohlen); and, 3dly, Those with an intumesced coke, (Back Kohlen).
These three denominations sufficiently indicate the aspect and mode of existence of each of the three sorts of coke, as well as the transition which may take place from one kind to the other.
In all these kinds of coal, as in unaltered vegetable fibres, the quantity of charcoal obtained, differs according as a slow or quick heat is employed during distillation. In general, this difference of product is so much the greater, that the coals contain less charcoal. The coals with intumesced coke, however, form an exception. These often, with a greater quantity of charcoal, present greater differences of product in the two modes of carbonization, than with a less quantity of charcoal the coals with pulverulent coke do, and especially than those with conglutinated coke. At the most, these differences of product, in all the va rieties of coal examined by M. Karsten, do not exceed 6 per cent., and even this maximum of difference was only observed in a coal with an intumesced coke, which presented a mean quantity of charcoal. The produce in coke of coals of this class, when they possess a greater quantity of charcoal, does not vary more than 4 per cent. in the two modes of carbonization.
Another remarkable fact is, that the application of a low heat, raised very slowly to the strongest red heat, diminished in coals the property of furnishing either a conglutinated or an intumesced coke. A coal which, on being subjected to a rapid incandescence, announces itself as belonging to the second class (coal with conglutinated coke), may, by means of a heat raised very slowly, present the aspect of a coal of the first class (that with pulverulent coke). It is chiefly in the transitions from the one to the other class, that this fact is observed. In like man
ner, by means of a slow heat, a coal of the third class presents the aspect of the second, and especially if the coal in question possesses only in a feeble degree the property of furnishing an intumesced coke. In every case, if the heat be produced but slowly, the swelling of the coals with vesicular coke is diminished. They then form a less loose, less bulky, and less light mass, than if an ardent heat had been rapidly applied.
A distinction between the coals which swell, and those which do not, has long been established in the arts, because these two kinds of combustibles act very differently. Manufacturers have readily observed the great influence which the manner that dif
ferent kinds of coal have in comporting themselves, exercises over their use. They have remarked, that the coals which swell cannot always be substituted by those which do not, and the reverse. But, between the one and the other, common opinion establishes no other difference than the following:-The coals, it is said, which swell, are only distinguished by a greater quantity of constituent parts, which are not carbonaceous, parts which have been designated by the name of Bitumen; in other words, it is the quantity of charcoal which decides whether a coal possesses the property of swelling or not.
This opinion is incorrect; and, so far from this being the case, it is most commonly observed, that the quantity of charcoal is greater in those coals which swell, than in others. There are coals of the first and second classes (with pulverulent and conglutinated coke), which, on being carbonised, do not yield more than about 50 per cent. of coke, and very few coals of the third class (with intumesced coke) yield so little. On the contrary, a great number of these coals with intumesced coke, furnish upwards of 80 per cent. of a very loose and swollen coke. Such a coal cannot contain so many constituent parts, which are not charcoal, as a coal with pulverulent or conglutinated coke, from which there is only obtained about 50 per cent. of coke.
The products of the distillation of coal in the dry way are well known. The greater the quantity of charcoal, the thicker is the consistence of the oil which is formed. All the varieties of coal, without exception, on being subjected to dry distillation, give feeble traces of ammonia. The coals with pulverulent coke, when they have a small proportion of charcoal, present traces of an acid. In all the varieties of coal belonging to this first class, the proportion which the aqueous fluid bears to the oily fluid, is greater than in those of the second class; and, in these latter, the proportion is greater than in the coals of the third class (those with intumesced coke). The quantity of gaseous substances, and of fluids or vapours which is formed, is in the inverse ratio of the contents in charcoal. A smaller quantity of gas is disengaged by the varieties of black coal, than by most of the brown coals; but, in the former, the combinations of carburetted hydrogen are more predominant. Sulphuretted hydrogen gas is only formed when the coal is mixed with iron pyrites, which it