86 
RADIATION THROUGH THE EARTH’S ATMOSPHERE. 
such person in support of any indictment for a misdemeanour at common law or other¬ 
wise, or of any proceeding under the provisions of this Act. 
6. This Act may be cited for all the purposes as “ The Exhibition Medals Act, 1863.” 
ON RADIATION THROUGH THE EARTH’S ATMOSPHERE. 
BY JOHN TYNDALL, ESQ., E.R.S. 
(Read at a Meeting of the Royal Institution of Great Britain^ Friday, Jan. 23, 1863.) 
Nobody ever obtained the idea of a line from Euclid’s definition that it is length 
without breadth. The idea is obtained from a real physical line drawn by a pen or 
pencil, and therefore possessing width; the idea being brought, by a process of ab¬ 
straction, more nearly into accordance with the conditions of the definition. So also 
with regard to physical phenomena ; we must help ourselves to a conception of the 
invisible by means of proper images derived from the visible, afterwards purifying our 
conceptions to the needful extent. Definiteness of conceptions, even though at some 
expense to delicacy, is of the greatest utility in dealing with physical phenomena. 
Indeed it may be questioned whether a mind trained in physical research can at all 
enjoy peace, without having made clear to itself some possible way of conceiving of 
those operations which lie beyond the boundaries of sense, and in which sensible phe¬ 
nomena originate. 
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 ni¬ 
trogen. These elementary atoms may be figured as small spheres scattered thickly in 
the space which thickly surrounds the earth. They constitute about 99 J 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 diverse atoms have coalesced to form little systems of 
atoms. The molecules of aqueous vapour, for example, consist of two atoms of hydro¬ 
gen 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 heated 
nothing but motion.” Well, we must figure this motion communicated to the medium 
in which the atoms swing, and sent in ripples through it with inconceivable velocity to 
the bounds of space. Motion in this form, unconnected with ordinary matter, but speed¬ 
ing through the interstellar medium, receives the name of Radiant Heat; and if com¬ 
petent to excite the nerves of vision, we call it Light. 
Aqueous vapour was defined to be invisible gas. "Vapour was permitted to issue 
horizontally with considerable force from a tube connected with a small boiler. The 
track of the cloud of condensed steam was vividly illuminated by the electric light. 
What was seen, however, was not vapour, but vapour condensed to water. Beyond the 
visible end of the jet the cloud resolved itself into true vapour. A lamp was placed 
under the jet at various points ; the cloud was cut sharply off’at that point, and when 
the flame was placed near the efflux orifice the cloud entirely disappeared. The heat 
of the lamp completely prevented precipitation. This same vapour was condensed and 
congealed on the surface of a vessel containing a freezing mixture, from which it was 
