378 Proceedings of the Royal Society. 



3. Qn the Property of Transmitting Light, possessed by 

 Charcoal and Plumbago, in fine plates and particles- 

 By John Davy, M.D., &c. 



The charcoal of the pith of the elder consists of plates of extra- 

 ordinary thinness. It was in examining this charcoal, that the 

 author first observed the property which is the subject of his paper. 

 He detected it by means of the microscope, using a high magnifying 

 power. By analogy, he was led to infer that the power of transmit- 

 tino- light must belong to charcoal in general, in all its varieties, 

 when reduced to the state of fine powder or filaments, — an influence 

 which he found confirmed by experiment in a number of different 

 instances, as the charcoal of the pith of the sycamore, of the pith of 

 the rush, the fibre of cotton, flax, &c. He also found it to belong to 

 lamp-black, to cork in very fine powder, to anthracite, and plumbago. 



The light transmitted he found to vary in its hues, from almost 

 white, as in the instance of the thinnest plates of the charcoal of the 

 pith of the elder, to brown and red of various shades, in tlie instances 

 of lamp-black, antliracite, and plumbago. 



He considers the property of translucency belonging to charcoal 

 and plumbago, in their finely divided state, as favourable to the 

 opinion now commonly received, that these substances and diamond 

 owe their marked peculiarities not to diff'erence of chemical mixture, 

 but of mechanical structure. Incidentally, he notices the specific 

 gravities of these substances, — stating, as the result of his own ex- 

 periments, that the specific gravity of charcoal, cork, and anthracite, 

 is about 1.5; and that of plumbago about the same, making allow- 

 ance for the ferruginous and earthy matter with which the carbon in 

 this mineral is mixed. 



In conclusion, he oflFers the conjecture, that the coloured tints of 

 vapour and fluids in which carbon is suspended, may be connected 

 with the translucency of this substance, and that other bodies, hither- 

 to considered opaque, may be found capable of transmitting light, 

 when examined in a manner similar to that which he has employed. 



January 23. — The Very Reverend Principal Lee, V.P., in 

 the Chair. 



1, Chemical Observations on the Flowers of the Camellia 

 Japonica, Magnolia grandiflora, and Chrysanthemum 

 Leucanthemum ; and on three proximate principles 

 which they contain. Part I. J^y Dr Hope. 



