OF TRANSMITTING LIGHT. 337 



It occurred to me as probable, that the very thin and delicate flower-leaves 

 of plants might yield charcoal of marked transparency. The petals, probably from 

 being more heterogeneous in composition than the substances before mentioned, 

 contract more when subjected to heat, and are more changed, than they are in 

 form. This may be in a great measure prevented, by confining them in the 

 process of carbonization between two surfaces of platina or silver-foil. Charcoal 

 thus obtained from the petal of the pansy, adhering to the foil, was sufficiently 

 translucent to allow the metallic splendour of the platinum to be seen beneath it. 

 Examined with a high magnifying power, it displayed no pores ; it appeared of a 

 bright brown colour, where most translucent, evidently the effect of transmitted 

 light reflected from the metal. 



I shall make mention of only one other variety of vegetable charcoal, that 

 made from oak-wood. This, when reduced by trituration to the most subtle 

 powder, spread on glass, and held between the eye and a bright light, obscures 

 the light in a certain degree, like soot, the matter of smoke from flame, and im- 

 parts to it a brownish red hue ; and under the microscope, like soot, each minute 

 particle appears to transmit a reddish light : the smallest particles barely within 

 the limit of distinct vision, using a glass with a magnifying power of nearly four 

 hundred diameters and a strong light, have a peculiar splendour, not unlike that 

 of the dust of the diamond.* 



As most animal substances enter into fusion in the process of carbonization, 

 it is not easy to obtain animal charcoal, fit for trial, of the kind under considera- 

 tion. However, such as I have tried, has exhibited pretty distinctly the same 

 property as that from vegetables, as regards the translucency of its minute parts. 

 I may mention particularly that from gold-beaters' skin, and that from silk. 



The charcoal of gold-beaters' skin displays with great clearness the texture 

 of this substance, provided it is prepared with care, so as to prevent the running 

 together of its parts in fusion under the action of heat which may be effected by 

 charring it, spread on glass or metallic foil. The structure it exhibits under a 

 high magnifying power, is, as it were, of two layers, very like that of gold leaf; 

 one composed of fibres forming an irregular open net-work, almost, if not quite 

 opaque ; the other, of a close tissue, either apparently homogeneous, or consist- 

 ing of extremely minute fibres, transmitting a brownish light, about equal in 

 strength to the green light transmitted by the finer tissue of the gold-leaf, the 

 coarser fibre of which reflects yellow light.f 



* 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. 



t Some parts of gold leaf, which, under a feeble illumination, reflect yellow light, with a stronger 

 transmit green. 



