and
of their vapours proved absolutely the same. There is no known
exception to this law; so that, to determine the position of a vapour
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 bearing. If the position of a liquid as an absorber
and radiator determine that of its vapour, the position of water fixes
that of aqueous vapour. 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 carbon 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 vapour 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 first six substances in the list of liquids examined, the
radiant and absorbent powers augment 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 hand, its molecule is rendered more
complex by the introduction of a new element. Benzol contains carbon
and hydrogen, while alcohol contains carbon, hydrogen and oxygen.
Thus, not only does atomic