THE KINETIC THEORY OF MATTER. 131 



means that the surface-particles of one body catch against the surface- 

 particles of the other, and that there are forces between these particles 

 against which the work is done. The collision will give the colliding 

 particles greater motion than that which they possessed before, and so 

 we get the idea that the heat developed is, at least in part, due to the 

 increase in kinetic energy of these particles. 



When a body is heated it generally expands i.e. its molecules are on 

 the average at a greater distance apart. This again implies that motion 

 has been given to them to move them apart : but it also probably implies 

 the existence of energy of position, the equivalent of the work done in 

 increasing the separation of the molecules against the forces between 

 them. 



We also know that a hot body radiates energy to the surrounding 

 bodies energy which affects our sense of sight, if the source be hot 

 enough. We know, too, that this energy is associated with waves, 

 implying a vibratory motion at the source. The waves are so small, and 

 the frequency of vibration producing them is so great, that we are led to 

 suppose the individual molecules or atoms to be themselves the vibrating 

 bodies. As to the mode of vibration we can only guess. It may be that 

 each separate atom vibrates in itself as a bell when struck or each atom 

 may travel to and fro about some mean position, as a bell on a spring 

 might do or both modes may co-exist. We may therefore have energy, 

 existing in various ways, associated with the atoms and molecules. We 

 may have vibrating energy of the atom, energy of motion of the atom as 

 a whole, energy of motion of the molecule as a whole, energy of separa- 

 tion of the atoms from each other, energy of separation of the molecules 

 from each other, and we must now regard heat as a mixture of some or 

 all of these. In other words, heat consists in the kinetic and potential 

 energies of the ultimate particles of matter. 



We must make further assumptions as to the forms and constitutions 

 of the atoms and the actions between them if we wish to go further and 

 work out their motions by ordinary mechanics. One attempt of this 

 kind was made in the ring-vortex theory of Lord Kelvin,* which sup- 

 posed that the atom was a ring-vortex of fluid existing in an infinite 

 fluid. According to this theory, the energy of any system is entirely 

 kinetic, either energy of motion of the vortices, or energy of motion of 

 the surrounding fluid. It would therefore reduce all forms of energy to 

 the single form of kinetic energy, and on this account it is worthy of 

 mention, though recent electrical work has led to the imagination of a 

 very different type of atom.f 



Without entering into any details as to the construction of the atoms 

 or molecules, we may still be able to form some general notions of their 

 arrangements and motions in the various states of matter. 



In solids, the atoms and molecules are probably only agitated about some 

 mean position, for we know that solids keep their shape for indefinitely 

 long periods if not exposed to external action. Carefully preserved jewels 

 engraved by the ancients still possess all their original sharpness of out- 

 line. We cannot, then, suppose that the molecules are travelling about, 

 but only moving to and fro to a very limited extent. The molecules 



* Maxwell's Scientific Papers, vol. ii. p. 470 ; " Atom," from Encyo. rit., 9th ed. 

 f J. J. Thomson, Electricity and Matter, p. 90. 



