294 FRAGMENTS OF SCIENCE. 



CHAPTER XVII. 



CONTRIBUTIONS TO MOLECULAR PHYSICS. * 



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 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 aggrega- 

 tion, 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 mole- 

 cules by the force of cohesion be of paramount influence, 

 then we may expect that liquids will exhibit a deportment 

 toward radiant heat altogether different from that of the 

 vapors from which they are derived. 



The first part of an inquiry conducted in 1863-64 was 

 devoted to an exhaustive examination of this question. 

 Twelve different liquids were employed, and five different 

 layers of each, varying in thickness from 0.02 of an inch 

 to 0.27 of an inch. The liquids were enclosed, not in glass 

 vessels, which would have materially modified the incident 

 heat, but between plates of transparent rock-salt, which 

 only slightly affected the radiation. The source of heat 

 throughout these comparative experiments consisted of a 

 platinum wire, raised to incandescence by an electric cur- 

 rent of unvarying strength. The quantities of radiant heat 

 absorbed and transmitted by each of the liquids at the 

 respective thicknesses were first determined. The vapors 

 of these liquids were subsequently examined, the quantities 

 of vapor employed being rendered proportional to the quan- 

 tities of liquid previously traversed by the radiant heat. 

 The result was that, for heat from the same source, the 

 order of absorption of liquids and of their vapors proved 



* A discourse delivered at the Royal Institution, March 18, 1864 

 Supplementing, though of prior dat*>, the Rede Lecture on Radiation. 



