XVII. 



CONTRIBUTIONS TO MOLECULAR PHYSICS* 



HAVING on previous occasions dwelt upon the 

 enormous differences which exist among gase- 

 ous 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 con- 

 densed to a liquid, or a liquid congealed to a solid, the 

 molecules 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 ether 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 



* A discourse delivered at the Royal Institution, March 18, 

 1864 supplementing, though of prior date, the Rede Lecture on 

 Radiation. 



