THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



JULY 1872. 



I. What determines Molecular Motion? — the Fundamental Pro- 

 blem of 'Nature, By James Croll, of the Geological Survey of 

 Scotland^. 



IT is an opinion which is daily gaining ground that at some 

 future time, perhaps not far distant, all the purely physical 

 sciences will be brought under a few general laws and princi- 

 ples. However wide and diversified physical phenomena may 

 seem at first sight, and however great and radical the apparent 

 distinction between the several sciences, yet to the eye of the 

 thoughtful physicist, who sees deeper into the subject, they 

 begin to appear as but the varied modifications of a few common 

 principles. For example, Heat, Electricity, and Magnetism are 

 in their ordinary phenomena very unlike each other ; yet modern 

 investigation has shown that they are mutually convertible. 

 Heat can be converted into Electricity, and Electricity into Mag- 

 netism. Magnetism can be converted into Electricity, and Elec- 

 tricity into Heat. This indicates that these corresponding 

 sciences are not radically distinct, that their phenomena have a 

 common origin, that in each we have the same force manifested 

 under different forms. 



To arrive at unity among the facts of nature has ever been, 

 and ever will be, the aim of physical investigation. We try to 

 induce a unity amongst the multifarious facts of the senses by 

 bringing as many of them under a certain conception as will 

 be rationally connected by it. But we soon find that we must 

 have a higher unity; and we endeavour to reduce the number of 



* Communicated by the Author. 

 Phil Mag. S. 4. Vol. 44 No. 290. July 1872. B 



