260 



KNOWLEDGE. 



[November 2, 1896. 



artificial cold. No heat, however, travels ; what travels is 

 a wave which, beating' upon the colder body, warms it. 

 It is not to bo thought that the mere neif,'libourhood of a 

 colder body causes waves to start from the hotter ; one 

 must suppose rather that the action is always going on, 

 but that the warming effects only become sensible when 

 one body is colder than the other. Tlie effect, too, i^ 

 reciproi;al, for the hotter body is cooled when a colder is 

 in its neighbourhood. It is almost obvious that a bojy 

 cannot go on miking waves which carry energy through 

 the eth >r without itself losing energy. In the neighbour- 

 hood of a similar but colder body the hotter loses more 

 energy than it receives from the colder until their tem- 

 peratures become equal, when the waves which they 

 mutually emit and absorb are of equal heating value and 

 there is no further change of temperature. Thus, as long 

 as a body has any heat at all, as long as the molecular 

 parts are dancing (whether rhythmically or with jostling), 

 the body sends out ether waves. When the temperature 

 of the body is being raised, vibrations of greater rapidity 

 are set up, while the slower movements do not die, but, 

 on the contrary, increase in amplitude as more energy is 

 put into the body. The slower movements begin first, 

 and at any given time theirs is the greater part of the 

 energy which the body possesses in virtue of its tempera- 

 ture. This is why the production of an intensely bright 

 light always involves the expenditure of so much energy. 

 The brightest rays, which have little energy, will only come 

 after the others have been produced, and they are also the 

 first to die out. 



As the slow succeeding pulses of ether emitted by a hot 

 body have more energy than the others, and as all the 

 pulses traverse ether at the same rate, one must coQclude 

 that the amplitude of the vibrations of the ether is greater 

 in the case of the slower pulses, much as in a storm those 

 waves whose crests are most widely separated are also 

 higher from trough to crest than the shorter waves which 

 have been formed later. The vibration in ether is different, 

 and is kept up in a different way from the swing of the 

 water in the waves of the sea. The free undulation in 

 water is maintained by mutual attraction between the 

 water and the earth, and by the inertia of the water, 

 which causes it in each half-swing to pass the position of 

 equilibrium. There are, however, other kinds of waves 

 in fluids which seem more nearly to resemble the ether 

 waves in the mechanism of vibration, namely, the elastic 

 waves, such as sound waves, which, when once set up, 

 run freely by the mutual action of the pirts of the 

 fluid. The particles jump forward a little way, forcing 

 themselves m between the particles in front. They 

 produce thus a compressed layer, which in its turn 

 compresses the next layer by a similar action, itself 

 quickly recovering from the pressure by the spring-back of 

 the particles. The loudness of a sound depends upoQ the 

 amount of compression, or upon the difference of pressure 

 between the condensed and the rarefied part. Amplitude in 

 the case of sound means, therefore, the amount of a c im- 

 pression, not the length of the excursion of a vibrating 

 particle. There is, again, another kind of elastic wave — for 

 there is an elasticity of shape as well as an elasticity of 

 volume. Fluids offer an elastic resistance to change of 

 volume, and can therefore transmit a wave of compression 

 or of rarefaction ; but change of shape calls out no elastic 

 resistance in fluids. It is otherwise with solids, which 

 spring back if de-formed and vibrate elastically, the shape 

 of the solid going through a repeating series of changes 

 of form, the vibrations diminishing quickly or slowly 

 according as the solid is more or less viscous. This kind 

 of vibration is transmitted by solids in the manner of a 



wave, as the following example shows. Let a long wire 

 or cord be hung vertically, the upper end being forcibly 

 clamped so that it cannot move, and a weight being 

 attached to the lower end so as to keep the wire taut. 

 If the weight — that is to say, the lower end of the wire — be 

 twisted, each part of the wire follows the twist. If the 

 substance be very rigid (as iron, for instance) the longi- 

 tudinal transmission of the effect of the twist is very 

 rapid. With a less rigid body — say, an india-rubber cord — 

 the parts lag more behind. If, now, the end of the wire or 

 cord be let go the end untwists, twists up again in the 

 reverse direction, untwists again, and so on for a longer or 

 shorter time according as the body is less or more viscous. 

 In twisting or in untwisting each part of the wire is a 

 certain definite time behind the lower end, for the effect is 

 transmitted from point to point at a definite speed which 

 is constant for each substance, and is the rata of trans- 

 mission of a wave of transverse displacement in that 

 substance. The rata is greater in more rigid bodies, for 

 rigidity is the transmitting force, but is diminished by 

 greater density, for the miss to be moved is proportional to 

 the density. 



Dismissing as inapplicable the case of waves, such as 

 those of the sea, which run by attraction to an external 

 body, we have to enquire whether ether waves, such as 

 produce the sensation of light, are elastic waves of longi- 

 tudinal displacement (like sound waves) or of transverse 

 displacement, or whether they are of both kinds '? The 

 fundamental experiment which shows that light is pro- 

 duced by waves is that of interference. If the light from 

 a point reach a screen by two paths of slightly different 

 length, a series of light and dark bands is produced upon 

 the screen where the rays overlap. This shows that two 

 portions of light may either give a double illumination or 

 no illumination, according to the difference of the distances 

 traversed by the two rays from their common starting 

 point. Where the vibrations of the ether are in the same 

 phase of motion the amount of motion is doubled, giving 

 increased illumination; where the vibrations are in opposite 

 phases there is no illumination. Longitudinal waves are 

 always capable of interference, but two parallel or coin- 

 cident waves of transverse displacement, of which the 

 vibrations are in (fixed) directions at right angles to one 

 another, could not interfere so as to give diminished 

 motion {i.e., dark binds), for no part of the motion of 

 vibration in one ray is at all in the direction of the 

 vibrations of the other. This case of non-interference has 

 been shown to occur in ether waves when subjected to the 

 treatment called "polarization." It is therefore concluded 

 that the waves of ether, which give heating, lighting, and 

 chemical effects, are waves of transverse displacement, 

 whence follows the corollary that ether resembles a solid 

 in possessing rigidity. The enormous velocity of light 

 waves, which is thousands of times greater than the rate 

 at which ordinary solids transmit wave motion, shows that 

 the rigidity of ether is extremely great in proportion to its 

 density. 



The intensity of light in a vacuum diminishes with the 

 square of the distance from a luminous point which is 

 radiating in all directions. Now the wave front in such a 

 case is a spherical shell whose centre is the luminous point. 

 The area of surface of the shell increases as the square of 

 the radius, hence it follows that the total illuminating 

 effect over the whole surface of the shell is always the 

 same whatever be its radius ; or, in other words, light 

 travels through ether without loss. This shows, within 

 the limits of experimental error, that ether is perfectly free 

 from viscosity, the wave motion undergoing no diminution 

 through any sort of frictional resistance. The energy 



