332 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



tion, coal in the steam-engine, zinc and sulphuric acid in the 

 electro-magnetic machine, food for the horse ; in the windmill 

 we use up the motion of the wind, which is arrested by the sails. 



Conversely, if we have a motive force at our disposal, we can 

 develop with it forms of action of the most varied kind. It 

 will not be necessary in this place to enumerate the countless 

 diversity of industrial machines, and the varieties of work which 

 they perform. 



Let us rather consider the physical differences of the possible 

 performance of a motive power. With its help we can raise 

 loads, pump water to an elevation, compress gases, set a railway 

 train in motion, and through friction generate heat. By its aid 

 we can turn magneto -electric machines, and produce electric 

 currents, and with them decompose water and other chemical 

 compounds having the most powerful affinities, render wires in- 

 candescent, magnetise iron, &c. 



Moreover, had we at our disposal a sufficient mechanical 

 motive force, we could restore all those states and conditions 

 from which, as was seen above, we are enabled at the outset to 

 derive mechanical motive power. 



As, however, the motive power derived from any given 

 natural process is limited, so likewise is there a limitation to the 

 total amount of modifications which we may produce by the use 

 of any given motive power. 



These deductions, arrived at first in isolated instances from 

 machines and physical apparatus, have now been welded into a 

 law of nature of the widest validity. Every change in nature 

 is equivalent to a certain development, or a certain consumption 

 of motive force. If motive power be developed, it may either 

 appear as such, or be directly used up again to form other changes 

 equivalent in magnitude. The leading determinations of this 

 equivalency are founded on Joule's measurements of the me- 

 chanical equivalent of heat. When, by the application of heat, 

 we set a steam-engine in motion, heat proportional to the work 

 done disappears within it ; in short, the heat which can warm a 

 given weight of water one degree of the Centigrade scale is able, 

 if converted into work, to lift the same weight of water to a 



