PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 495 



atoms adapted themselves easily to mathematical treatment. Dalton 

 (ill 1803) introduced his atomic hypothesis into modern physics, which 

 siuce then has been subject to different views on the constitution of 

 matter, and therefore has had to undergo various modifications. On 

 the basis of certain physical and chemical manifestations, it is a gener- 

 ally accepted proposition that matter consists of unchangeable particles, 

 or atoms separated by spaces, which, compared to their own size, are 

 very considerable. We must assume as many kinds of atoms as there 

 are of chemically indivisible substances. A group of similar or dissimi- 

 lar atoms considered as connected by chemical affinity, according to 

 certain laws, constitutes a molecule. Molecules are chemically divisible, 

 mechauically, however, indivisible. Atoms then are the components of 

 molecules ; an atom is neither mechanically nor chemically divisible. 

 Certain phenomena lead to the conclusion that an atom cannot exist in 

 an isolated condition, so that even in the chemical elements at least 

 two atoms are combined in a molecule. Chemical processes extend to 

 the separation of the atoms constituting a molecule, and to the uuion 

 of materially different atoms in the molecules of compound substances. 

 In the chemical separation the stronger chemical elective attraction of 

 one substance mostly takes from the molecules of the other the chemi- 

 cally affinitive atoms, whereby the molecules of the bodies in question 

 are decomposed. Under favorable conditions the separated dissimilar 

 atoms combine, by virtue of their chemical affinity for each other, to 

 form molecules of a chemically new compound body. 



While ehemical affinity dominates in the atoms of molecules, physical 

 forces of attraction (cohesion, adhesion) are in action between the mole- 

 cules of bodies and of the earth, just as that force called the force of 

 gravity or gravitation prevails between the celestial bodies. 



Matter then consists of mechauically indivisible molecules; the latter 

 again can only be divided into their own atoms. The comparatively 

 large spaces intervening between the atoms we assume to be filled with 

 an excessively fine and highty elastic substance, called aether, which 

 surrounds atoms and molecules alike in the manner of an atmosphere. 

 This aether exercises an attraction on the atoms and molecules of bodies, 

 while it repels its own particles in such wise that the density of the 

 aether surrounding each atom and molecule increases from without 

 towards the interior. A further assumption is that this aether is not 

 only distributed through the interstices or pores of bodies, but extends 

 throughout the entire universe. From the oscillations of the atoms of 

 this aether, transverse in direction to that of the propagation of the rays 

 of heat and light, the diffusion of the waves of heat and light is deduced, 

 and it has been proved by well-established experiments and irresistible 

 conclusions that the rays of heat and light are identical, and that they 

 only differ in the number of their oscillations and their effects. 



We will now examine in what way the permeability of certain sub- 

 stances by heat rays is explained on the one side, and the increase of 



