CONCLUDING EEMAEKS. 189 



as take electricity as an example. Suppose a pound of mer 

 cury at 400 be employed to produce a thermo-electric cur 

 rent, and the latter be in its turn employed to produce me* 

 chanical force ; if this latter force be greater than that which 

 the direct effect of heat would produce, then it could by com 

 pression raise the temperature of the mercury itself, or of a 

 similar quantity equally heated, to a higher point than its 

 original temperature, the 400 to 401, for example, which 

 is obviously impossible ; nor, if we admit force to be inde 

 structible, can it produce less than 400, or cool the second 

 body except by some portion of it being converted into 

 another form or mode of force. 



But as the mechanical effect here is produced through the 

 medium of electricity, and the mechanical effect is definite, 

 so the quantity of electricity producing it must be definite 

 also, for unequal quantities of electricity could only produce 

 an equal mechanical effect by a loss or gain of their own 

 force into or out of nothing. The same reasoning will apply 

 to the other forces, and will lead, it appears to me, necessa 

 rily and inevitably to the conclusion, that each force is defi 

 nitely and equivalently convertible into any other, and that 

 where experiment does not give the full equivalent, it is be 

 cause the initial force has been dissipated, not lost, by con 

 version into other unrecognised forces. The equivalent is the 

 limit never practically reached. 



The great problem which remains to be solved, in regard 

 to the correlation of physical forces, is this establishment of 

 their equivalents of power, or their measurable relation to a 

 given standard. The progress made in some of the branches 

 of this inquiry has been already noticed. Viewed in their 

 static relations, or in the conditions requisite for producing 

 equilibrium or quantitative equality of force, a remarkable 

 relation between chemical affinity and heat is that discovered 

 in many simple bodies by Dulong and Petit, and extended to 

 compounds by Neumann and Avogadro. Their researches 



