438 FRAGMENTS OF SCIENCE. 



XVII. 



CONTRIBUTIONS TO MOLECULAR PHYSICS. 1 



HAVING- on previous occasions dwelt upon the 

 enormous differences which exist among gaseous 

 bodies both as regards their power of absorbing and 

 emitting radiant heat, I have now to consider the effect 

 of a change of aggregation. When a gas is condensed 

 to a liquid, or a liquid congealed to a solid, the mole- 

 cules coalesce, and grapple with each other by forces 

 which are insensible as long as the gaseous state is 

 maintained. But, even in the solid and liquid con- 

 ditions, the luminiferous aether still surrounds the mole- 

 cules : hence, if the acts of radiation and absorption 

 depend on them individually, regardless of their state of 

 aggregation, the change from the gaseous to the liquid 

 state ought not materially to affect the radiant and 

 absorbent power. If, on the contrary, the mutual 

 entanglement of the molecules by the force of cohesion 

 be of paramount influence, then we may expect that 

 liquids will exhibit a deportment towards radiant heat 

 altogether different from that of the vapours from which 

 they are derived. 



The first part of an enquiry conducted in 1863-64 

 was devoted to an exhaustive examination of this ques- 

 tion. Twelve different liquids were employed, and five 



1 A discourse delivered at the Eoyal Institution, March 18, 1864 

 supplementing, though of prior date, the Rede Lecture on Radiation. 



