Prof, Challis on Newton's "Foundation of all Philosophy" 289 



to, or the explanations receive any consideration, so long as the 

 minds of physicists are preoccupied with modes of philosophizing 

 totally at variance with that which I have adopted. For this 

 reason I propose to conclude this communication with a brief 

 exposition of the principles of the antagonistic methods. 



The prevailing physical theories may be classed under two 

 heads, sufficiently distinguished in their general characteristics. 

 In the one class, the atomic constitution of all substances is ad- 

 mitted ; the existence of the aether is recognized ; certain move- 

 ments of the atoms, both of the aether and of the grosser bodies, 

 relatively to each other, and about axes they are supposed to 

 have, are assumed; dynamical effects are attributed to these 

 motions ; an undefined use is made of the word " polarity ; " 

 and the application of differential equations is in a great mea- 

 sure dispensed with. An example of this method of philoso- 

 phizing is exhibited by the work on l Matter and iEther/ cri- 

 ticized in the Number of this Magazine for last April, which 

 shows most remarkably how easy it is to invent causes of 

 phenomena, if all reference to an independent foundation of 

 philosophy, such as that insisted upon by Newton in Hegula 

 III., be omitted, and the necessity of deduction by mathematical 

 reasoning be consequently avoided. Theorizers of this class 

 have recourse to personal conceptions, which they form ad 

 libitum, and which, undoubtedly, are to be ranked among the 

 " somnia " that Newton refers to. Also these hypotheses are 

 generally such as require explanation quite as much as the 

 phenomena they are supposed to explain, and therefore do not 

 advance scientific knowledge a single step. 



The hypothesis of the isotropic constitution of the aether, 

 invented to account for phenomena of light, is very closely 

 allied to the class of hypotheses just spoken of. If it should be 

 more successful than it has as yet been in explaining pheno 

 mena, it w r ould still require to be itself explained. Having 

 already so often expressed my views on the undulatory theory 

 of light, I will only remark here that the necessity for the 

 isotropic hypothesis has arisen from the adoption of a theory 

 which does not recognize the fundamental ideas of matter and 

 force, and consequently does not regard the aether as a con- 

 tinuous fluid. 

 \ The other class of physical theories, which is now much in 

 vogue, is characterized by the deduction of generalizations from 

 special facts and laws. The most complete exhibition of this 

 method that I am acquainted with is contained in the article " On 

 Energy" in < Good Words' (Oct. 1862), to which I have before 

 referred in a communication " On the Source and Maintenance of 

 the Sun's Heat" (Phil. Mag. for June 1863). Professors Thom- 



Phil. Mag. S. 4. Vol. 26. No. 175. Oct. 1863. U 



