TAIT'S " THEHMODYXAMIC8. ' 



In iU obvious and triot sense no axiom can be more irrefragable. Even 

 in the hypothetical process, the impossibility of which it was intended to 

 avert, erery communication of heat is from a warmer to a colder body. 

 When the beat is taken from the cold body it flows into the working substance 

 which is at that time still colder. The working substance afterwards becomes 

 hot, not by communication of heat to it, but by change of volume, and when 

 it communicates heat to the hot body it is itself still hotter. 



It is therefore hardly correct to assert that heat has been transmitted or 

 transferred from the colder to the hotter body. There is undoubtedly a transfer 

 of energy, but in what form this energy existed during its middle passage is 

 a question for molecular science, not for pure thermodynamics. 



In a note added in 1864 Clausius states the principle in a modified form, 

 " that heat cannot of itself pass from a colder to a warmer body " * and finally, 

 in the new edition of his Tlieory of Heat (1876) he substitutes for the words 

 "of itself" the expression "without compensation f." 



With respect to the first of these emendations we must remember that 

 the words "of itself" are not intended to exclude the intervention of any kind 

 of self-acting machinery, and it is easy, by means of an engine which takes 

 in heat from a body at 200* C., and gives it out at 100 to drive a freezing 

 machine so as to take heat from water at 0, and so freeze it, and also a 

 friction break so as to generate heat in a body at 500. It would therefore 

 be necessary to exclude all bodies except the hot body, the cold body, and 

 the working substance, in order to exclude exceptions to the principle. 



By the introduction of the second expression, " without compensation," com- 

 bined with a full interpretation of this phrase, the statement of the principle 

 becomes complete and exact ; but in order to understand it we must have a 

 previous knowledge of the theory of transformation-equivalents, or in other 

 words of entropy, and it is to be feared that we shall have to be taught 

 thermodynamics for several generations before we can expect beginners to receive 

 as axiomatic the theory of entropy. 



Thomson, in his "Third Paper on the Dynamical Theory of Heat" (Trcuis. 

 R. S. Edin. xx. p. 265, read March 17, 1851), has stated the axiom as follows : 



1 DIM die Warrae nicht von selbst aus einem kalteren in einem warmeren Korper tiber- 

 gehen bum. 



\ Ein Warmedbergang aus einem kalteren in einem warmeren Korper kann nicht ohne Com- 

 penmtion Suit finden. 



