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SCIENCE 



[N. S. Vol. XXXVI. No. 924 



when its importance was more fully recog- 

 nized, Clausius gave it the name of entropy, 

 and established the important property 

 that its total quantity remained constant 

 in reversible heat exchanges, but always 

 increased in an irreversible process. Any 

 process involving a decrease in the total 

 quantity of entropy was impossible. Equiv- 

 alent propositions with regard to the possi- 

 bility or impossibility of transformations 

 had previously been stated by Lord Kelvin 

 in terms of the dissipation of available 

 energy. But, since Carnot's solution had 

 been overlooked, no one at the time seems 

 to have realized that entropy was simply 

 Carnot's caloric under another name, that 

 heat could be measured otherwise than as 

 energy, and that the increase of entropy in 

 any irreversible process was the most ap- 

 propriate measure of the quantity of heat 

 generated. Energy so far as we know must 

 always be associated with something of a 

 material nature acting as carrier, and 

 there is no reason to believe that heat 

 energy is an exception to this rule. The 

 tendency of the kinetic theory has always 

 been to regard entropy as a purely abstract 

 mathematical function, relating to the dis- 

 tribution of the energy, but having no phys- 

 ical existence. Thus it is not a quantity of 

 anything in the kinetic theory of gases, but 

 merely the logarithm of the probability of 

 an arrangement. In a similar way, some 

 twenty years ago the view was commonly 

 held that electric phenomena were due 

 merely to strains in the ether, and that 

 the electric fluids had no existence except 

 as a convenient means of mathematical ex- 

 pression. Recent discoveries have enabled 

 us to form a more concrete conception of 

 a charge of electricity, which has proved 

 invaluable as a guide to research. Per- 

 haps it is not too much to hope that it may 

 be possible to attach a similar conception 



with advantage to caloric as the measure 

 of a quantity of heat. 



It has generally been admitted in recent 

 years that some independent measure of 

 heat quantity as opposed to heat energy is 

 required, but opinions have differed widely 

 with regard to the adoption of entropy as 

 the quantity factor of heat. Many of these 

 objections have been felt rather than ex- 

 plicitly stated, and are therefore the more 

 difficult to answer satisfactorily. Others 

 arise from the difficulty of attaching any 

 concrete conception of a quantity of some- 

 thing to such a vague and shadowy mathe- 

 matical function as entropy. The answer 

 to the question "What is caloric?" must 

 necessarily be of a somewhat speculative 

 nature. But it is so necessary for the ex- 

 perimentalist to reason by analogy from the 

 seen to the unseen, that almost any answer, 

 however crude, is better than none at aU. 

 The difficulties experienced in regarding 

 entropy as a measure of heat quantity are 

 more of an academic nature, but may be 

 usefully considered as a preliminary in at- 

 tempting to answer the more fundamental 

 question. 



The first difficulty felt by the student in 

 regarding caloric as the measure of heat 

 quantity is that when two portions of the 

 same substance, such as water, at difEerent 

 temperatures are mixed, the quantity of 

 caloric in the mixture is greater than the 

 sum of the quantities in the separate por- 

 tions. The same difficulty was encountered 

 by Carnot from the opposite point of view. 

 The two portions at different temperatures 

 represented a possible source of motive 

 power. The question which he asked him- 

 self may be put as follows: "If the total 

 quantity of caloric remained the same when 

 the two portions at different temperatures 

 were simply mixed, what had become of the 

 motive power wasted?" The answer is 

 that caloric is generated, and that the 



