498 
ON THE ABSORPTION AND RADIATION 
OF HEAT. 
BY H. DEBUS, Ph.D., F.R.S. 
On the absorbing energy of aqueous vapour, and its in- 
fluence on terrestrial radiation, we quote Professor TyndalPs 
own words : — 
When we speak of radiation through the atmosphere, we ought to be able 
to affix definite physical ideas, both to the term atmosphere and the term 
radiation. It is well known that our atmosphere is mainly composed of the 
two elements, oxygen and nitrogen. These elementary atoms may be figured 
as small spheres scattered thickly in the space which immediately surrounds 
the earth. They constitute about 99| per cent, of the atmosphere. Mixed 
with these atoms, we have others of a totally different character ; we have 
the molecules, or atomic groups, of carbonic acid, of ammonia, and of 
aqueous vapour. In these substances divers atoms have coalesced to form 
little systems of atoms. The molecule of aqueous vapour, for example, con- 
sists of two atoms of hydrogen united to one of oxygen ; and they mingle 
as little triads among the monads of oxygen and nitrogen, which constitute 
the great mass of the atmosphere. These atoms and molecules are separate ; 
but in what sense ? They are separate from each other in the sense in which 
the individual fishes of a shoal are separate. The shoal of fish is embraced 
by a common medium, which connects the different members of the shoal, 
and renders intercommunication between them possible. A medium also 
embraces our atoms ; within our atmosphere exists a second and a finer 
atmosphere, in which the atoms of oxygen and nitrogen hang like suspended 
grains. This finer atmosphere unites not only atom with atom, but star with 
star ; and the light of all suns and of all stars is, in reality, a kind of music 
propagated through this interstellar air. This image must be clearly seized, 
and then we have to advance a step. We must not only figure our atoms 
suspended in this medium, but we must figure them vibrating in it. In this 
motion of the atoms consists what we call their heat. “ What is heat in us,” 
as Locke has perfectly expressed it, “ is in the body heating nothing but 
motion.” Well, we must figure this motion communicated to the medium in 
Avhich the atoms swing, and sent in ripples through it with inconceivable 
velocity through space. Motion in this form, unconnected with ordinary 
matter, but speeding through the interstellar medium, receives the name 
of radiant heat, and, if competent to excite the nerves of vision, we call it 
light. 
Tbe waves of beat speed from our earth through our atmo- 
sphere towards space. These waves dash in their passage 
against the atoms of oxygen and nitrogen, and against the 
