through the Earth's Atmosphere. 201 



phenomena ; we must help ourselves to a conception of the in - 

 visible by means of proper images derived from the visible, after- 

 wards purifying our conceptions to the needful extent. Definite- 

 ness of conceptions, even though at some expense to delicacy, is 

 of the greatest utility in dealing with physical phenomena. In- 

 deed 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 atmo- 

 sphere and the term radiation. It is well known that our atmo- 

 sphere 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 sur- 

 rounds 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 diverse atoms have coalesced to form little 

 systems of atoms. The molecule of aqueous vapour, for example, 

 consists 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 to 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 ordi- 

 nary matter, but speeding through the interstellar medium, 



