different degrees of a high temperature, is of some importance, even in an economical point of view. From the same dil may be obtained for lighting, either a larger quantity of gas of bad quality, or a smaller quantity of incomparably better gas, according as the carbonization is effected by means of a weaker or stronger heat.

If the principal object of the operation were to obtain charcoal, it would be necessary to employ at first as low a heat as possible, and not to make it rise till near the end, in order to lose only the smallest possible quantity of charcoal in the gaseous combinations and fluids which are formed. This also shews that the products of dry distillation, with reference to the same organic body, must present differences as well of quantity as of kind, according as the temperatures employed have been different. This is a circumstance which, in a great number of cases, would require to be more taken into consideration than it has hitherto been.

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It is known that the products of the distillation of unaltered and perfectly dry vegetable fibres in the air, are an empyreumatic acid, water, oil, a very small quantity of alcoholic substance, and a gaseous mixture, consisting of carbonic acid gas, carbonic oxide gas, carburetted hydrogen gas, and olefiant gas. The mutual relation of all these combinations, and the quantity of carbonaceous residuum, depend upon the temperature.

If shavings of wood be exposed for a long time to a temperature which does not rise above 120° of Reaumur, a period arrives when there is no longer observed any change of weight. In this operation, wood dried at the temperature of the air, but not at the temperature of boiling water, loses from 66 to 69 per cent. of its weight. Dried at the latter temperature, the wood would lose at the most from 56 to 59. Thus the residuum, which perfectly resembles common wood-charcoal, only that it presents a somewhat duller aspect, weighs from 41 to 44 per cent. of the real quantity of wood which has been employed, allowance being made for moisture. This carbonaceous substance is what M. de Rumford has named the frame-work, or skeleton, of plants. That philosopher considered it as a pure charcoal, which he imagined to exist in equal quantity in all plants. But M. Karsten concludes, from his own researches, that the pre

tended skeleton of plants is only an imperfectly decomposed vegetable fibre, and that it is not at all a pure charcoal.

In reality, says M. Karsten, vegetable fibres, after the disunion of their elements, preserve the external form of undecomposed fibres, and they experience no other change in their form than a diminution of size; but it is a consequence of the fact which has been mentioned; it is because the disunion of the elements of these vegetable fibres, at a temperature of about 120° of Reaumur, cannot be carried beyond a loss of weight which varies from 66 to 69 per cent. There results from this, that, if the temperature be raised above that point, then a new loss of weight commences, which, in its turn, remains constant for the new degree, until, at length, at the temperature of incandescence, the disunion of the elements of these fibres is completely effected; and after this no diminution of weight takes place.

The products of this slow decomposition are very different from those which are obtained by a decomposition effected by a rapidly increased heat. Wood of hornbeam (Carpinus betulus), which, under a rapid carbonization, yields the ordinary products of distilled wood, and furnishes 13.3 per cent. of charcoal, developes, under a slow elevation of the temperature, much more water, carburetted hydrogen gas, and carbonic acid gas. It then furnishes 26.1 per cent. of charcoal, that is to say, nearly twice as much as in the case of a rapid carbonization. The decomposition of unaltered vegetable fibres commences, therefore, at a pretty low temperature; and the reason of this is, that, in wood-fibres, the quantity of oxygen and hydrogen, as is known by the analyses of MM. Gay Lussac and Thenard, occurs pretty nearly in the relation necessary for the formation of water.

The charcoal obtained from vegetable fibre by means of dry distillation, or by carbonization, appears to vary but little in our common woods. In a synoptical table, the author presents the results of experiments upon twenty-one kinds of unaltered vegetable fibres, such as oak, beech, hornbeam, birch, pine, lime, straw, fern, reed, and a piece of birch-wood which had served as a prop in a mine for an hundred years, but was still in good preservation. In all these trials, the matter was employed in the state of shavings, which had been perfectly dried in the open

air, at a temperature of from 12° to 15° of Reaumur. The same species of matter was, on the one hand, submitted to a very rapid carbonization, for which, from the commencement of the distillation, an incandescent heat was employed; and, on the other hand, to a temperature which was made to rise very slowly to this point. The contents in ashes were carefully determined, by means of the incineration of charcoal under the muffle of an assay furnace. The weight of the ashes is deducted from that of the charcoal in the following table.

It is sufficient to cast a glance upon this table to observe a general result, which is as follows:-Whatever difference the vegetable fibres of gramineæ, ferns, and different species of wood, present to the eye, these matters all afford nearly equal quantities of charcoal by dry distillation. The differences which are observed here and there, may arise from the impossibility of

* Instead of Old Birch, say Birch-wood, which, for upwards of 100 years, had been used as a support in a mine, and was still in good preservation.

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 degree, that a stronger current of air must be employed, than the proper nature of the combustible, without this circumstance, would require. Hence the great difficulty of employing this substance advantageously for the purpose in view. The ashes of fossil wood and brown coal contain no traces of fixed alkali. Silica, alumina, oxide of iron, sulphate of lime, a little lime