174 



SCIENTIFIC THOUGHT. 



48. 

 Kelvin's 

 available 



energy. 



available energy as distinguished from total energy had 

 been introduced by Lord Kelvin and by Maxwell. This 

 free energy is measured not only by the heat liberated, 

 but depends on all the other factors, such as volume and 

 pressure, the number of chemical substances engaged, and 

 their physical conditions. The doctrine of energy and the 

 conception of free energy pointed out a method of co- 

 ordinatincr all these different factors and reducing them 

 to a common measure. As Rankine, by the introduction 

 of the term potential energy, did much to clear the ideas 

 and guide the reasoning in dynamical science, so Helni- 

 holtz, by introducing the term free energy, did a great 

 deal to introduce into chemical science the fruitful con- 

 ceptions which had been elaborated and appHed in phys- 

 ical research. The term free or available energy seems 

 to describe more naturally the characteristic property of 

 all energy which is useful for doing w^ork, whilst the 

 opposite term entropy — which measures the unavailable 

 or hidden energy — refers to a quantity for which we have 

 no immediate means of perception.^ 



auce of these somewhat abstruse 

 expositions lies mainly in two 

 directions : First, in the recog- 

 nition of the fact that for the cor- 

 rect description of natural pheno- 

 mena and changes the knowledge 

 of the total energy is as little suf- 

 ficient as that of the total weight 

 or mass, but that it -is necessary 

 to introduce the conception of use- 

 ful energy, of energy which is free 

 or available for doing ■work ; 

 secondly, in the recognition that 

 the course of chemical changes or 

 reactions cannot be measured by 

 attending to one special property, 

 such as weight, or temperature, or 

 «utropy, but that it requires the 



measurement of a quantity which 

 comprises all the different agencies 

 in nature, this quantity being the 

 energy of the system or substances 

 in question and its availability. A 

 third point, which is of more or less 

 importance according to the general 

 view adopted, is this, that the ma- 

 thematical formula8 involved have 

 exhibited the analogy between 

 chemical and mechanical processes, 

 the latter being those which were 

 earliest and are most easily grasped 

 by the mind. 



^ As Prof. Ostwald has remarked, 

 it is to a great extent a matter of 

 taste what particular form one 

 adopts out of the many in which the 



