ATOM. 



on ,0 Average, any one bell is not again struck till it has ceased to 

 vibrmtr, then the audible result will appear a continuous sound, composed of 

 UM ouud emitted by bells in all states of vibration, from the clang of the 

 stroke to the final hum of the dying fundamental tone. 



Hut now let the number of bells be reduced while the same number of 

 stroke* are given in a second. Each bell will now be struck before it has 

 oeawd to vibrate, so that in the resulting sound there will be less of the 

 fundamental tone and more of the original clang, till at last, when the peal is 

 reduced to one bell, on which innumerable hammers are continually plying 

 their strokes all out of time, the sound will become a mere noise, in which 

 no inimical note can be distinguished. 



In the case of a gas we have an immense number of molecules, each of 

 which is set in vibration when it encounters another molecule, and continues 

 to vibrate as it describes its free path. The molecule is a material system, 

 the parts of which are connected in some definite way, and from the fact that 

 the bright lines of the emitted light have always the same wave-lengths, we 

 learn that the vibrations corresponding to these lines are always executed in 

 the same periodic time, and therefore the force tending to restore any part of 

 the molecule to its position of equilibrium in the molecule must be propor- 

 tional to its displacement relative to that position. 



From the mathematical theory of the motion of such a system, it appears 

 tliat the whole motion may be analysed into the following parts, which may 

 be considered each independently of the others : In the first place, the centre 

 of mass of the system moves with uniform velocity in a straight line. This 

 velocity may have any value. In the second place, there may be a motion of 

 rotation, the angular momentum of the system about its centre of mass re- 

 maining during the free path constant in magnitude and direction. This angular 

 momentum may have any value whatever, and its axis may have any direction. 

 In the third place, the remainder of the motion is made up of a number of 

 component motions, each of which is an harmonic vibration of a given type. 

 In each type of vibration the periodic time of vibration is determined by the 

 nature of the system, and is invariable for the same system. The relative 

 amount of motion in different parts of the system is also determinate for each 

 type, but the absolute amount of motion and the phase of the vibration of 

 each type are determined by the particular circumstances of the last encounter, 

 and may vary in any manner from one encounter to another. 



