PHYSICAL SCIENCE. 13 



to the electrical current, and thereby affording fresh proof 

 of the reciprocity of actions alluded to above. One form 

 of force always disappears as another is evolved. 



We might give, had we space for them, many other curious 

 instances of this reciprocity of relation, as manifested by the 

 several forces of heat, electricity, magnetism, mechanical 

 power, and chemical affinity. One we may select, as an 

 example of beautiful contrivance as well as striking results. 

 By a certain combination of apparatus, in which light, acting 

 through the daguerreotype, was the initiating force, Mr. 

 Grove obtained first the chemical action upon the plate; 

 thence a current of electricity circulating through wires ; next 

 magnetism by a coil of these wires ; then the production of 

 heat, testified by the delicate helix of Breguet ; and finally, 

 motion, as shown by the needles of the galvanometer. In- 

 stances of this kind have rapidly multiplied, since the co- 

 relation and convertibility of forces has been recognised as 

 a principle and applied to research. They are derived not 

 solely from recent experiment, but even more frequently and 

 fruitfully from phenomena already familiar to us as facts, 

 but which awaited for their illustration the happy induction 

 now attained. 



The beauty of this principle, however, is not limited to the 

 expression of the reciprocity or mutual convertibility of the 

 physical forces with which we are dealing. There is every 

 reason to believe in a further co-relation as regards their 

 equivalents of power or measurable quantitative effects. 

 Though this generalisation is still not complete, numerous 

 cases occur where it is thoroughly attested by the results of 

 experiment. The discoveries of Faraday have furnished some 

 of the most striking examples of constant quantitative relation 

 between electrical power and chemical actions and changes. 

 The researches of Dulong, Petit, and Neumann, show very 

 remarkable relations between chemical affinity and heat, 



