CONCLUDING REMARKS. 171 



ideas ; if we ever do so, our mental powers must undergo a 

 change of which at present we see no prospect. 



If we apply to any other force the mode of reasoning 

 which in the course of this Essay has been applied to heat, we 

 shall arrive at the same conclusion, and see that a given source 

 of power can, supposing it to be fully utilised in each case, 

 yield no more by employing it as an exciter of one force than 

 of another. Let us take electricity as an example. Suppose 

 a pound of mercury at 400 be employed to produce a thermo- 

 electric current, and the latter be in its turn employed to pro- 

 duce mechanical force ; if this latter force be greater than that 

 which the direct effect of heat would produce, then it could 

 by compression or friction raise the temperature of the mer- 

 cury itself, or of a similar quantity equally heated, to a higher 

 point than its original temperature, the 400 to 401, for ex- 

 ample, which is obviously impossible; nor, if we admit force 

 to be indestructible, can it produce less than 400, 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, necessarily and 

 inevitably to the conclusion, that each force is definitely and 

 equivalently. convertible into any other, and that where expe- 

 riment does not give the full equivalent, it is because the initial 

 force has been dissipated, not lost, by conversion into other 

 unrecognised forces. The equivalent is the limit never prac- 

 tically 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 enquiry has been already noticed. Viewed in their 

 static relations, or in the conditions requisite for producing 



