ATOM. 463 



turbance of the luminiferous medium, when analysed by the prism, will be 

 found to contain, besides the part due to the regular vibrations, other motions, 

 depending on the starting and stopping of each particular vibrating body, 

 which will become manifest as a diffused luminosity scattered over the whole 

 length of the spectrum. A spectrum of bright lines, therefore, indicates that 

 the vibrating bodies when set in motion are allowed to vibrate in accordance 

 with the conditions of their internal structure for some time before they are 

 again interfered with by external forces. 



It appears, therefore, from spectroscopic evidence that each molecule of a 

 rarefied gas is, during the greater part of its existence, at such a distance 

 from all other molecules that it executes its vibrations in an undisturbed and 

 regular manner. This is the same conclusion to which we were led by con- 

 siderations of another kind at p. 452. 



We may therefore regard the bright lines in the spectrum of a gas as 

 the result of the vibrations executed by the molecules while describing their 

 free paths. When two molecules separate from one another after an encounter, 

 each of them is in a state of vibration, arising from the unequal action on 

 different parts of the same molecule during the encounter. Hence, though the 

 centre of mass of the molecule describing its free path moves with uniform 

 velocity, the parts of the molecule have a vibratory motion with respect to 

 the centre of mass of the whole molecule, and it is the disturbance of the 

 luminiferous medium communicated to it by the vibrating molecules which 

 constitutes the emitted light. 



We may compare the vibrating molecule to a bell. When struck, the 

 bell is set in motion. This motion is compounded of harmonic vibrations of 

 many different periods, each of which acts on the air, producing notes of as 

 many different pitches. As the bell communicates its motion to the air, these 

 vibrations necessarily decay, some of them faster than others, so that the 

 sound contains fewer and fewer notes, till at last it is reduced to the funda- 

 mental note of the bell*. If we suppose that there are a great many bells 

 precisely similar to each other, and that they are struck, first one and then 

 another, in a perfectly irregular manner, yet so that, on an average, as many 

 bells are struck in one second of time as in another, and also in such a way 



* Part of the energy of motion is, in the case of the bell, dissipated in the substance of the bell 

 in virtue of the viscosity of the metal, and assumes the form of heat, but it is not necessary, for the 

 purpose of illustration, to take this cause of the decay of vibrations into account. 



