Prof. Challis on the Principles of Theoretical Physics, 315 



perly belongs, was not yet discovered. Turning now to other 

 departments of physics — the phenomena of light, heat, electri- 

 city, galvanism, magnetism, and diamagnetism, — it may, in the 

 first place, be stated that with respect to all these theory is in a 

 condition analogous to that of physical astronomy in the time of 

 Kepler. Experiment has established the existence of a great 

 number of facts and laws, which are only so many problems 

 that wait for solution by some theoretical generalization. The 

 process for effecting such generalization must be of the same 

 kind as that which has been so successful in physical astronomy* 

 Some hypothesis, or hypotheses, suggested by the antecedents 

 of physical science, must be thought of, and be made the basis 

 of appropriate calculation, in order that the truth of the hypo- 

 theses may be tested by comparison of the results of the calcu- 

 lation with experimental facts. 



I am not aware that any general physical theory, supported 

 by mathematical reasoning, and comprehensive of all the physi- 

 cal forces, has hitherto been proposed, excepting that of which 

 I have given an outline in communications made from time to 

 time to this Magazine. In the supplementary Number for last 

 June I have expressed the intention of going through a revision 

 of the proofs of certain propositions in hydrodynamics which are 

 essential to the general theory. Before, however, carrying out 

 this intention, I propose in this communication, in the first 

 place, to re-state the fundamental hypotheses of the theory, for 

 the purpose of showing that, in conformity with the principles 

 above laid down, they have regard to antecedent physical science, 

 and are proper for being the basis of mathematical calculation. 

 Then I shall endeavour to indicate in general terms the process 

 to be followed in order to found on these hypotheses a general 

 physical theory, and the requirements which such a theory must 

 satisfy. 



The hypotheses are of two distinc tkind — sone relating to the 

 agency by which the physical forces act, and the other to the 

 qualities of the ultimate constituents of material substances. 

 These two classes must be regarded separately, as they require 

 the application of different tests, and admit of different degrees 

 of verification. 



The principal hypothesis of the first class is, that a very rare 

 and elastic fluid, called the aether, of uniform elasticity through- 

 out, pervades all space not occupied by the atoms of visible and 

 tangible substances, and that under all circumstances its pres- 

 sure is proportional to its density. The theory does not recog- 

 nize any active force which is not resident in this medium. 



The conception of a medium of much greater tenuity and 

 elasticity than the air is of very long standing, and was enter- 

 tained especially by Descartes, who even applied it to account 



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