338 DR DAVY ON THE PROPERTY OF CHARCOAL AND PLUMBAGO 



Silk, eveii when charred, compressed between pieces of silver-foil, in conse- 

 quence of fusion, loses entirely its fibrous structure, affording a hard, brittle, char- 

 coal.* A single fibre of silk, however, may be preserved in its filamentous form, 

 if charred with a graduated heat on a plate of glass ; when it appears as a glassy 

 thread, expanded at intervals into globules, of different degrees of transparency, 

 and of different shades of brown, according to their thinness. 



Plumbago, in a very attenuated state, like charcoal, appears to be translucent. 

 The powder of it, rubbed on glass so as to render dim its surface, like smoke, im- 

 parts to light transmitted through it a certain colour, a brownish hue ; and seen 

 with a high magnifying power, exhibits much the same appearance as the fine pow- 

 der of the charcoal of oak-wood already mentioned ; its minute particles transmit a 

 reddish light, and are very brilliant when seen with a strong light. The streak of 

 plumbago on glass, when very light, exhibits the same colour, and admits of a 

 line underneath it being pretty distinctly seen. The specimens I have examined 

 have been a foliated kind, which occurs in small quantity, disseminated through 

 the dolomite rock of Ceylon, associated with ceylanite, the common plumbago of 

 commerce, sold in the state of powder, and two pieces of the foliated kind from 

 Cumberland. The Ceylon specimen appears to be very pure ; it admits of exten- 

 sion under pressure, or, in other words, of a certain degree of malleability,! and, 

 also, of having its minute fragments united by pressure, as it were by a process of 

 welding. 



Coke and anthracite, reduced to a very fine powder, I find, in regard to the 

 transmission of light, resemble the powder of charcoal of oak-wood ; the minute 

 particles have the same brilliant appearance under the microscope. Two speci- 

 mens of the former were tried, one obtained from bituminous coal, the other from 

 compact lignite ; these substances themselves differ very little from the coke they 

 yield in the degree of transparency of their minute particles. One specimen only 

 of anthracite was made the subject of experiment. Strong illumination was re- 

 quired to shew the translucency of its minute particles ; many of the minute 

 fragments, having flat surfaces, reflected white light. 



* 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 tho 

 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, confervas, and others of the cryptogamia ; 

 one species that I have thus tried (Byssus globosd) displays its structure in a very distinct manner, 

 composed of beaded fibres of about y^gg inch in diameter. 



t 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. 



