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the proportion of azote is so small, in all the varieties of brown coal and black coal that have been submitted to examination by Mr Karsten, that this substance does not appear to be an essential constituent part of them.
Several brown coals and black coals yield an acid liquor by distillation; but most kinds of black coal furnish none. Peat, in the dry distillation, furnishes so great a quantity of acid water, that it is difficult to recognise clearly in that substance the ammoniacal basis which occurs in it, and this even on saturating the acid with potash.
Mr Karsten has carefully investigated and described the very different effects which are produced, whether on wood, and, in general, on unaltered vegetable fibre, or on altered vegetable fibres, on peat, brown coal, and black coal, by the different chemical re-agents, such as water, alcohol, sulphuric ether, caustic ammonia, hydrosulphuret of ammonia, nitric acid, and concentrated sulphuric acid. In his work we even find detailed accounts of the processes followed in these investigations. We shall confine ourselves, however, to the principal results. Those which are obtained on making the acids act upon vegetable fibres, whether altered or recent, are perfectly in accordance with the manner in which acids comport themselves, and the circumstances of the body upon which they act. Nitric acid, which is easily decomposed, and, from this very circumstance, capable of oxidising, produces more promptly, and in a higher degree, the oxidation of vegetable fibres. This acid changes them into a substance analogous to tannin, or even into an acid, while sulphuric acid can only operate a conversion of the fibres into gum, and finally into sugar. Unaltered fibre undergoes its metamor phoses more quickly and more completely, because the greater proportion of the quantity of oxygen and hydrogen to the quantity of carbon facilitates the action of acids.
In proportion as the quantity of carbon increases, the chemical effect of acids becomes more and more feeble, and perfectly pure charcoal appears no longer susceptible of alteration from acids, excepting in a single case, which happens when this substance occurs, as it does in wood-charcoal, in a loose state of mechanical aggregation.
Glance coal, graphite and diamond resist the action of acids;
and this perhaps solely on account of their great density. Díamond, which is the densest kind of charcoal known, only burns at a very high temperature, and by means of pure oxygen. Glance coal and graphite are incomparably more easy to be destroyed; and the charcoal which is obtained on distilling black coal, brown coal, and unaltered vegetable fibres in the dry way, burns the more readily the looser the state of aggregation it assumes during the process of carbonization, or the less the quantity of carbon the body contains which has been employed for producing the charcoal. A coal that is carbonised in a furnace, or still better in a close vessel, affords a charcoal much more compact and more difficult to be burnt, than that which comes from the same coal carbonised in the open air.
Elevation of temperature causes a decomposition of the combustible, and the formation of new combinations. This process has received the name of carbonization, because in this operation the residuum consists of pure charcoal. If hydrogen, oxygen and carbon, on being subjected to different degrees of temperature, also obey different laws of combination, the quantity of pure charcoal which remains after the carbonization, must depend, not only upon the state of the body which is to be carbonised, but also upon the different degrees of temperature which have been employed during this operation. This is actually what takes places. Several resins and fats, which contain much more carbon than vegetable fibres, leave no trace of charcoal in their spontaneous decomposition at a high temperature; and in the same vegetable fibre, the quantity of charcoal residuum depends entirely upon the degree of heat employed during the carbonization.
It is not the quantity of the carbonaceous residuum alone that must vary according to the different degrees of the temperature employed. The same cause must render more variable still the quantity and condition of the other combinations which are formed during distillation in the dry way, that is to say, during carbonization. This is the case precisely, because the quantity of the charcoal residuum is but a consequence of the nature and condition of the gaseous combinations and fluids, or vapours, which are formed during the operation. This difference in the manner in which organic combinations are affected under the
different degrees of a high temperature, is of some importance, even in an economical point of view. From the same oil 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.
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.
SUBMITTED TO CARBONIZATION.
QUANTITIES OBTAINED FROM
100 PARTS OF WOOD.
By rapid carbo
By slow carbonization.
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.