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Published online by Cambridge University Press: 17 January 2013
I am not aware that this property has yet been known to belong to these substances; they are commonly considered and spoken of as opaque, without any qualification.
It was in examining the charcoal of the pith of the elder, that I was first led to entertain doubt of the accuracy of the current opinion.
The pith of the elder consists of polyhedral cells, commonly pentagonal, of from about 
to 
inch in diameter, formed of woody matter of extraordinary fineness, as may be inferred from their transparency when seen under the microscope, and their great lightness. They are unaltered in form, when converted into charcoal. The charcoal obtained (that which I examined was from a shoot of this year gathered in December) was brilliant, as might be expected, from its consisting of plates, and very soft and brittle; in other respects, in mass, it was nowise peculiar, having the ordinary colour and opacity of charcoal. When broken up, however, and seen with a high magnifying power, the detached plates were found to be transparent in different degrees (allowing lines drawn on the glass-support to be seen under them), and of different shades of brown—passing into black on one hand, and into almost white on the other, especially as seen by reflected light. In general appearance they were not unlike mica viewed with the naked eye. No pores were visible in them; but in some there were foramina, circular, or oval, varying in diameter from about 
of an inch to 
. The plates themselves varied in size from about 
to 
of an inch, estimating their width, and selecting the most entire. So thin were they, that, under a glass magnifying 800 diameters, the most transparent had no apparent thickness; the darker, less transparent, may have had a thickness of from about 
to 
of an inch, judging from one, the edge of which, when floating in water, was so inclined as to offer a tolerable view of it.
page 336 note * Mr Bauer, in the account he has given of the microscopical appearance of cotton, appended to the “History of the Cotton Manufacture in Great Britain,” by E. Baines jun., Esq., describes the twisted appearance of the fibres as being often owing to the junction and torsion of two fibres: this I have never witnessed, and I am induced in consequence to question the correctness of the observation; Mr Bauer may have been deceived by using a microscope of indifferent construction.
page 337 note * It may be remarked that this powder, by reflected light under the microscope, appears white, but by transmitted, highly coloured; and when not in exact focus, almost black. This applies to the finest powder, viz. that just within the limits of distinct vision, using a lens magnifying about 400 diameters, as well as to powder somewhat coarser.
page 337 note † Some parts of gold leaf, which, under a feeble illumination, reflect yellow light, with a stronger transmit green.
page 338 note *
So very different in appearance is the charcoal of cotton, linen, and silk under the microscope, that the admixture of either in a fabric is more easily recognised after charring than before, especially in the instance of a mixed fabric of silk and linen that has been in use,—the coarser fibres of both being of nearly the same diameter, and, after wearing, the jointed appearance of the fibre of linen becoming very indistinct. The process of charring, I may add, may probably be employed with advantage in examining the minute structure of many of the lower vegetables, such as the byssi, confervæ, and others of the cryptogamia; one species that I have thus tried (Byssus globosa) displays its structure in a very distinct manner composed of beaded fibres of about 
inch in diameter.
page 338 note † It is right to remark, that I first heard of this property belonging to the purer forms of plumbago from Professor Jameson. Is it not owing to this quality that plumbago exhibits a metallic lustre when rubbed? The compact kind, when broken by main force, is without this lustre, is of a dull opaque black, not unlike fractured basalt, but on the slightest friction it acquires the lustre of a metal.
page 339 note * Whilst the diamond is comparatively of the high specific gravity 3.5, I find that of charcoal, coke, and anthracite (making allowance for the ash yielded by the latter) is only about 1.5. This is the result of some trials made with considerable care. The method employed was briefly the following. In the instance of the charcoal, whilst hot from the crucible in which it had been prepared, it was weighed in air; then, with distilled water, it was subjected to the air-pump, till it sunk and ceased to give out any air, when it was weighed in water; after which it was dried, ignited, and again weighed in air.
A piece of charcoal of the oak, weighing 12 grs., thus treated, appeared to be of sp. gr. 1.519 at 53° F.; a piece of charcoal of deal, weighing 5.67 grs., of sp. gr. 1.54, and reduced to powder of the sp. gr. 1.45.
In the instances of anthracite and coke, the same method was used, omitting the weighing in air the second time, after the weighing in water, as they yielded nothing soluble in water, and ascertaining the quantity of ash, or foreign fixed matter, which each afforded on incineration.
A portion of a specimen of anthracite, for which I was indebted to Professor Jameson, weighing 65.2 grs., appeared to be of the sp. gr. 1.57; it contained 4.7 per cent. of ash. A portion of coke, weighing 18.02 grs., obtained from bituminous coal, appeared to be of the sp. gr. 1.70; it contained 6.8 per cent. of ash. The ash of the anthracite consisted chiefly of silica, with a little alumina, coloured light red by peroxide of iron; the ash of the coke, of silica, with only a trace of alumine and peroxide of iron, of the latter not sufficient to colour it; both were without lime or alkali. The difficulty of extracting the whole of the air from the anthracite, charcoal, and coke, was considerable, especially from the coke. After three days' exposure to the action of the air-pump, the effect was produced on the anthracite; in about the same time on the charcoal; but not in less than eleven on the coke; it floated eight days,—and this notwithstanding that the pump was frequently worked—the mercury in the gauge standing steadily, after the first day, at .25 inch, and although the total quantity of air to be disengaged was equal only to the volume of 4.48 grs. of water. In one instance, the charcoal was boiled in distilled water, after it had ceased to give out air under the exhausted reservoir; but without effect in increasing its specific gravity.
The specific gravity of plumbago is stated to range between 1.987 and 2.456. A specimen of the compact kind, from Borrowdale, I found of sp. gr. 2.264, and after the exhaustion of adhering air, by the air-pump, 2.316. A specimen of the foliated kind, I found of the sp. gr. 2.22, and after having been subjected to the action of the air-pump, 2.26. The former yielded, on incineration, 11.48 per cent. of ash, retaining the form of the mass, of a light ochre yellow, which was found to consist chiefly of silica, with a little peroxide of iron, and a very little alumine, with a trace of lime and magnesia. The latter being incinerated with great difficulty, was deflagrated in red hot nitre; it yielded 4 per cent. of ash, which was found to consist chiefly of silica and peroxide of iron, with a little lime and magnesia. The ash was in the form of a powder of fine scales. If the iron and silicon exist in plumbago uncombined with oxygen, their presence may account for the specific gravity of the mineral exceeding that of charcoal and anthracite; but, if combined with oxygen, then it must be admitted that the carbon in plumbago is in a denser state than in charcoal. The circumstance that plumbago is slightly magnetic is in favour of the first idea, and also the fact, as I have ascertained, that it yields air (which it may be presumed is hydrogen) when acted on by dilute sulphuric acid previously purged of air by the air-pump, and after which iron may be detected in solution.