CONTRIBUTIONS TO MOLECULAR PHYSICS. 295 
absolutely the same. There is no known exception to this 
law; so that, to determine the position of a vapor as an 
absorber or a radiator, it is only necessary to determine the 
position of its liquid. 
This result proves that the state of aggregation, as far 
at all events as the liquid stage is concerned, is of altogether 
subordinate moment a conclusion which will probably 
prove to be of cardinal importance in molecular physics. 
On one important and contested point it has a special bear- 
ing. If the position of a liquid as an absorber and radiator 
determine that of its vapor, the position of water fixes that 
of aqueous vapor. Water has been compared with other 
liquids in a multitude of experiments, and it has been 
found, both as a radiant and as an absorbent, to transcend 
them all. Thus, for example, a layer of bisulphide of car- 
bon 0.02 of an inch in thickness absorbs 6 per cent., and 
allows 94 per cent, of the radiation from the red-hot 
platinum spiral to pass through it; benzol absorbs 43 and 
transmits 57 per cent, of the same radiation; alcohol absorbs 
67 and transmits 33 per cent., and alcohol, as an absorber 
of radiant heat, stands at the head of all liquids except 
one. The exception is water. A layer of this substance, 
of the thickness above given, absorbs 81 per cent., and 
permits only 19 per cent, of the radiation to pass through 
it. Had no single experiment ever been made upon the 
vapor of water, its vigorous action upon radiant heat 
might be inferred from the deportment of the liquid. 
The relation of absorption and radiation to the chemical 
constitution of the radiating and absorbing substances was 
next briefly considered. For the six substances in the list of 
liquids examined, the radiant and absorbent powers aug- 
ment as the number of atoms in the compound molecule 
augments. Thus, bisulphide of carbon has 3 atoms, 
chloroform 5, iodide of ethyl 8, benzol 12, and amylene 15 
atoms in their respective molecules. The order of their 
power as radiants and absorbents is that here indicated, 
bisulphide of carbon being the feeblest and amylene the 
strongest of the six. Alcohol, however, excels benzol as 
an absorber, though it has but 9 atoms in its molecule; 
but, on the other ha'nd, its molecule is rendered more com- 
plex by the introduction of a new element. Benzol con- 
tains carbon and hydrogen, while alcohol contains carbon, 
hydrogen and oxygen. Thus, not only does atomic multi- 
