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.