June 14, 1883] 



NA TURE 



149 



their fellows. This we know to be the case with all moving 

 masses of malter. Their independent motion is gradually over- 

 come by friction, and their motion brought into accordance with 

 that of surrounding mas-es. The same principle applies to 

 molecules. Friction, or molecular impact, its equivalent, must 

 quickly reduce their discordant movements, and bring all the 

 molecules of a mass inio harmonious motor relations. 



Molecules are related to their fellows in three distinct methods, 

 those of the solid, the liquid, and the gas. We can only con- 

 jecture the character of these relations. In the solid there is 

 perhaps no impact of molecules under normal conditions. Each 

 molecule may lie in the centre of a nest of attractions, within 

 which it describes a vibratory movement, without coming into 

 contact with any of the similarly moving molecules that surround 

 it. In the gas attraction also acts, bat not vigorously enough to 

 restrain the moving molecule, and cause vibration. Here, then, 

 impact is inces-ant. Yet the molecule can seldom escape from 

 its nest. It is driven backwards on all sides, and held captive 

 within a contracted space. A certain harmony must arise 

 between the motions of these gas molecules under conditions of 

 equalised temperature, which must tend to produce an equal 

 resistance to escape in every direction, and to confine each with- 

 in a fixed space in relation to tho-e surrounding. Such is not 

 the case when in one gas a volume of a different gas is set free. 

 The molecules, moving with different speeds, cannot harmonise 

 in their impacts. The molecules of the second gas find open 

 spaces in the net of the first, and rapidly disseminate themselves. 

 But the probability continues that in every homogeneous gas, at 

 a fixed temperature, there is little or no fiee excursion of mole- 

 cules. In the liquid there are also probably harmonious relations 

 of molecular motion. The character of this motion we do not 

 know. It is possibly a rotation around general centres of gravity. 

 However that be, each molecule must, under homogeneous con- 

 dition*, move within fixed limits. 



If such be the case, each molecule of a homogeneous mass 

 occupies a fixed field, from which it cannot of itself escape, and 

 whose boundaries it cannot change. These boundaries are 

 absolutely fixed by two energies. One of these is the momentum 

 of the molecule, by which it drives back those surrounding it. 

 The other is the pressure bearing upon the mass of which it 

 forms a part. This pres-ure is usually very great, so that the 

 space occupied by each molecule is very minute, and its change 

 of direction is necessarily very frequent. Tbe pressure coi.s'i- 

 tutes a tension, to which every molecule is subjeced, and each 

 has a normal rate of vibration, in accordance with this tension. 

 Of course the weight of the molecule is a constant element in 

 determining its vibratory pitch, which is therefore dependent 

 upon the fixed element of « eight and the varying element of 

 pressure. 



Such may be the condition of equilibrium of all material 

 molecules, whatever their state of aggregation, namely, the 

 confinement of each molecule within a limited field, within 

 which it incessantly moves, but from which it cannot escape. 

 And to this condition of absolute localisation of every particle 

 all matter tends to come, according to the theory of dissipation 

 of energy. But as nature now exists there are opposing in- 

 fluences which constantly overcome the tendency to locali ation. 

 One of these is the attraction of gravitation, which cau-es inde 

 pendent motion both in masses and in molecules. The other is 

 the heterogeneity of momentum of molecules, or what is usually 

 known as inequality of temperature. 



So far as the first of these is concerned, matter is now nearly 

 in equilibrium. The spheres are yet contracting, under the in- 

 fluence of gravity, but this contraction is so gradual as not to 

 materially affect the relations of molecules. Their mutual 

 localisation is but slightly disturbed by this cause. The inequality 

 of temperature is a more vigorous disturbing cause. To this are 

 chiefly due those transfers of energy through space, and of matter 

 through other matter to which all life and activity must be 

 ascribed. The inequality here referred to, as mentioned in my 

 former paper, is not of absolute heat contents, but of tempera- 

 ture, which is a very different thing, since density affects the 

 heat-containing powers of matter, and two masses of different 

 density may be equal in temperature while very unlike in heat 

 contents. 



The accepted view of radiation is that there is everywhere an 

 unceasing outward transfer of molecular motion, that each mole- 

 cule constantly yields and constantly receives radiant heat, 

 changing in temperature when these transfers are unequal, but 

 not when they are equal. This seems to me an incorrect view of 



the subject. If two masses of matter of equal temperature he i.i 

 contact there can be no radiation at all, instead of a double radia- 

 tion. For no molecule can transfer any of its energy to others. If 

 two molecules of equal momentum come into contact, neither can 

 lose nor gain momentum. The momentum of each remains the 

 same after as before the contact, and there cannot properly be said 

 to have been a mutual transfer of energies. The only change that 

 takes place is a change of direction of motion, and in this respect 

 the change in the one case balances that in the other. 



Thus properly we can speak of a transfer of motor energy 

 only when the momentum; of the molecules differ, and in this 

 case the transfer is from the most to the least vigorous only. 

 Heat is yielded outwardly, but not cold. This transfer is con- 

 tinually taking place, since the temperature of matter is very far 

 from a state of equilibrium. The transfer takes plaee in two 

 methods. One of these is through direct collision. The other i. 

 through vibratory impulse. We must consider these in succession. 

 Collision constantly takes place between the molecules of 

 gases. It takes place also in solids and liquids when by any 

 cause their molecular equilibrium is disturbed. The result of a 

 transfer of energy in this manner is what we may call an impact 

 radiation. Motion cannot lose or gain speed or change in direc- 

 tion except through the influence of counteracting energies. Thus 

 every impact radiation must run directly outward until overcome 

 by opposing energies of equal vigour. It is transferred from 

 particle to particle, its speed changing in accordance with the 

 weight of the particles, but its momentum continuing unchanged. 

 Such impulses are constantly travelling in all directions. They 

 are very frequently checked by equal opposite impulses, and 

 thus become local motion of molecules. 1 'his i, the ever-acting 

 equilibriating tendency. 



The other mode of molecular transfer is that supposed to be 

 through exchange of vibrations — the tadiation of li-,ht and heat. 

 This transfer pr sents two relations. One is that of speed. This 

 depends not on the speed of the motion, as in impact transfer, 

 but on tbe tension of the conveying particles. As their tension 

 increa-e-', the radiant wave is conveyed more rapidly. As it 

 diminishes, the wave travels more slowly. It is quite possible, 

 indeed, that the wave- of light may move with a different speed 

 in interastral space from their known rapidity in the ether of the 

 solar system, since ether may not be everywhere in the fame 

 state of tension. If so, certain astronomical conceptions would 

 be affected. This equal .-peed of radiant transfer, whatever the 

 rapidity of the vibration, indicates thatradiancediffersesentially 

 from impact transfer. In fact, there is no special neces ity in 

 their character that transverse vibrations should be transferred. 

 '1 hey may cease with any particle, and continue to exist as con- 

 tinuous vibra ion of that particle, or the energy of the vibration 

 be yielded to it as direct motion. If in this case the particle 

 move more vigorously than those surrounding, the vibratory 

 transfer will be replaced by im act transfer. 



This cessation of radiant transfer is constantly taking place. 

 Every wave of light and heat that comes to the earth's surface 

 is partly convened into local heat, partly transmitted through 

 transparent substances and partly repelled from the surfaces of 

 substances. Thus radiant transfer seems to be rather an accident 

 than a necessity of matter, since the energy thus transferred can 

 be immediately exchanged into 1 ici] energy, without the agency 

 of equal opposing energy, as in impact transfer. Whether the 

 wave shall travel onwards, be absorbed, or be repelled, appears 

 to depend on the tension of the substance which it affects. 

 Each molecule of every mass has its normal pitch of vibration, 

 in accordance with its weight and tension. If the radiant vibra- 

 tion be in complete accord with that pitch it will be retained as 

 local heat vibration. If in imperfect accord it will be partly 

 held. If discordant it will be transferred or repelled. In the 

 latter case it probably follows the easiest channel. Although 

 the direction of the ray is readily changed, yet probably it has 

 a special vigour of movement in the direct channel. Other 

 things being equal, it would follow the direct in preference to 

 the reflected course. Therefore in cases of reflection there must 

 be a resistance in the molecules of the reflecting substance 

 which makes it easier for the wave to change its direction than 

 to force itself on these molecules. This change of direction in 

 the wave, however, is not a change in the direction of motion. 

 The vibration continues in its original plane. It can only be 

 changed f om this plane by special influences within transparent 

 substances, in w hich the w ave vibrations, while acting upon the 

 molecules of the substance, are in some way distorted by their 

 interaction with the normal molecular motions. 



