458 W. R, Grove, Esq. m [Jan. 29, 



WEEKLY EVENING MEETING, 

 Friday, January 29. 



Sir Benjamin Collins Brodie, Bart., D.C.L. F.R.S. 



Vice-President, in the Chair. 



William Robert Grove, Esq. Q.C. V.P.RS. 

 On Molecular Impressions hy Light and Electricity, 



The term molecule is used in different senses by different authors : 

 by some it is employed with the same meaning as the word atom, 

 Le., to signify an ultimate indivisible particle of matter ; by others 

 to signify a definite congeries of atoms forming an integral element 

 of matter, somewhat as a brick may be said to be a congeries of 

 particles of sand, but a structural element of a house. 



The term is used this evening to signify the particles of bodies 

 smaller than those having a sensible magnitude, or only as a term of 

 contradistinction from masses. If there be any distinctive charac- 

 teristic of the science of the present century as contrasted with that 

 of former times, it is the progress made in molecular physics, or the 

 successive discoveries which have shown that when ordinary ponder- 

 able matter is subjected to the action of what were formerly called 

 the imponderables, the matter is molecularly changed. The re- 

 markable relations existing between the physical structure of matter, 

 and its effect upon heat, light, electricity, magnetism, &c., seems, 

 until the present century, to have attracted little attention : thus, to 

 take the two agents selected for this evening's discourse. Light and 

 Electricity, how manifestly their effects depend upon the molecular 

 structure of the bodies subjected to their influence? Carbon in 

 the form of diamond transmits light but stops electricity. Carbon 

 in the form of coke or graphite, into which the diamond may be 

 transformed by heat, transmits electricity but stops light. All solid 

 bodies which transmit light freely, or are transparent, are non- 

 conductors of electricity, or may be said to be opaque to it ; all the 

 best conductors of electricity, as black carbon and the metals, are 

 opaque or non-conductors of light.* Bodies which have a peculiar 

 but definite and symmetrical structure, such as crystals, affect light 

 definitely and in strict relation to their structure : witness the effects 

 of polarized light on crystals ; and there are not wanting instances 



* It should be borne in mind that these terms are not absolute, but only 

 express a high degree of approximation. 



