g94 FRAGMENTS OF SCIENCE. 
CHAPTEE 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 lo-ng 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 date, the Rede Lecture on Radiation. 
