320 Prof. Challis on the Principles of Theoretical Physics. 



hold the constituent atoms of bodies in equilibrium. Between 

 these forces and the force of galvanism there is, as experiment 

 shows, a close relation, which the theory, if true, will account 

 for. But clearly it does account for a relation by merely sup- 

 posing that the two kinds of forces are modifications of the dy- 

 namical action of the same aetherial medium. I may even go 

 further, and state, as a result to which my investigations point, 

 that while the forces of aggregation depend on the square of the 

 velocity in vibrations, galvanic force depends on the square of the 

 velochy in currents. Moreover, it is matter of experience that 

 galvanic, electric, magnetic, and diamagnetic forces have some 

 bond of connexion ; and obviously this circumstance also may be 

 referred to their being modes of action of the same medium. My 

 researches indicate, further, that these forces are all expounded 

 by the dynamic action of aetherial currents, and that they differ 

 from each other only in the conditions and circumstances under 

 which the currents are generated. Lastly, there is yet another 

 physical force, the relations of which to an aetherial medium, and 

 to other modes of force, are not readily made out : I mean the 

 force of gravity. If, however, all the other forces are modifica- 

 tions of aetherial pressure, it is reasonable to suppose that this 

 one is of the same kind. I have ventured to reason on this sup- 

 position, and have attempted to deduce (I think with success) 

 the known laws of gravity from the dynamical action of aetherial 

 waves of much larger magnitude than those which correspond to 

 molecular forces. It will be seen from these explanations that 

 very large demands are made on the hypothesis of a universal 

 aether, so large, indeed, that it seems impossible to account for 

 its meeting them in any degree excepting on the supposition that 

 it is a reality. 



From the foregoing discussion, one general inference of an 

 important character may be drawn. If the principles of the 

 proposed theory be admitted, it will follow that, previous to the 

 theoretical explanation of a vast number of facts and laws which 

 modern experiments have discovered, it will be necessary to 

 investigate, by mathematical reasoning applied to the aether, the 

 modes of its action under given circumstances. To illustrate 

 this remark, I may refer to the problem of the generation and 

 permanence of the sun's heat, and to that of the development of 

 the tails of comets, both of which have recently attracted the 

 attention of theorists. Now, as I conceive, neither of these 

 problems can be at all approached without the antecedent pos- 

 session of a mathematical theory of the force of heat, such as 

 that which I have deduced from the properties of an elastic 

 medium. If this course were pursued, it might perhaps be 

 found to be unnecessary to suppose that the sun's heat is main- 



