236 Mb KELLAND, ON THE MOTION OF 



Instances offer themselves in the cases of sound and light, since 

 both have, for a long period, been referred to vibrations, though the 

 difference in the nature of these vibrations had to be pointed out 

 before it was admitted that a comiilete parallel was not to be expected 

 between them. The same observation is applicable to the theories of 

 light and heat. Remarkable as are the analogies between them, 

 demanding as it would seem from their very nature the same mode 

 of explanation in each, there are nevertheless peculiarities in the latter 

 which seem to strike at the very foundation of the theory, and to 

 require the construction of another on totally new principles. 



On whatever grounds then a theory be raised, we must not be 

 discouraged if some succeeding facts appear for the moment to militate 

 against it, and particularly, when that theory is one in which the 

 action of force in its different modifications plays a conspicuous part, 

 for there we are presented with a range so wide that facts, almost 

 antagonist to each other, are brought together in the interpretations 

 of the various kinds of motion which occur. 



In the present memoir, I have ventured to push these interpreta- 

 tions to a considerable extent, from a conviction that the explanation 

 of many phenomena is contained in them, and tlie hope that in some 

 cases at least the real explanation may coincide with, or at any rate 

 bear a close resemblance to, those which I have attempted. 



I have adopted the hypothesis that the medium, whose motion we 

 have under consideration, is not composed of particles of one nature, 

 but of a regularly distributed series of particles of two kinds, of 

 which each is endued with forces and inertia differing from those of 

 the other. For the sake of distinction, in forming the equations, I 

 have called these media A and B, which when applied to- sound 

 signify air and vapour, when applied to light and heat, ether and 

 caloric or those substances, by whatever names they may be designated, 

 which serve for the propagation of these respectively. 



We will assume that the law of force is the same in both cases, 

 an assumption which there appears no reason to suspect. 



