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XXII. On the Property belonging to Charcoal and Plumbago, in fine Plates and Par- 

 tides, of Transmitting Light. By JOHN DAVY, M.D., F.R.S. L. & E., Inspector- 

 General of Army Hospitals, L. R. 



(Read 9th January 1843.) 



I AM not aware that this property has yet been known to belong to these sub- 

 stances ; 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 ggg^h to soo^h mcn m diameter, formed of woody matter of extraor- 

 dinary 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 jg^oo^h f an mcn * 4o^oth. The 

 plates themselves varied in size from about gggth to lo^gth 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 goioooth * 

 3o*oooth 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. 



That these plates consisted of charcoal, and were not composed of foreign 

 adventitious matter, such as silica, potassa, or carbonate of lime, I satisfied myself 

 by a few simple experiments. They were unaltered in the dilute mineral acids, 

 took fire when heated, and left hardly a perceptible ash : they deflagrated with 

 chlorate of potash, like common charcoal, and, in brief, did not appear to possess 

 any chemical qualities connected with their attenuated state, different from those 



VOL. xv. PART m. 4 x 



