82 Prof. Clausius on the Internal Work 



tity in such a way, that in treating of it we are obliged in some 

 measure to trust to probabilities ; whereas the external work is 

 immediately accessible to observation and measurement, and 

 thus admits of more strict treatment. Accordingly, since, in 

 my former paper, I wished to avoid everything that was hypothe- 

 tical, I entirely excluded the internal work, which I was able to 

 do by confining myself to the consideration of circular processes 

 — that is to say, operations in which the modifications which the 

 body undergoes are so arranged that the body finally returns to 

 its original condition. In such operations the internal work 

 which is performed during the separate modifications, partly in 

 a positive sense and partly in a negative sense, neutralizes itself, 

 so that nothing but external work remains, for which the prin- 

 ciple in question can then be demonstrated with mathematical 

 strictness, starting from the above-mentioned fundamental pro- 

 position. 



I have delayed till now the publication of the remainder of my 

 theorem, because it leads to a consequence which is considerably 

 at variance with the ideas hitherto generally entertained of the 

 heat contained in bodies, and I therefore thought it desirable to 

 make still further trial of it. But as I have become more and 

 more convinced in the course of years that we must not attach 

 too great weight to such ideas, which in part are founded more 

 upon usage than upon a scientific basis, I feel that I ought to 

 hesitate no longer, but to submit to the scientific public the 

 theorem of the equivalence of transformations in its complete form, 

 with the principles which attach themselves to it. I venture to 

 hope that the importance which these principles, supposing them 

 to be true, possess in connexion with the theory of heat will be 

 thought to justify their publication in their present hypothetical 

 form. 



I will, however, at once distinctly observe that, whatever he- 

 sitation may be felt in admitting the truth of the following 

 principles, the conclusions arrived at in my former paper, in refer- 

 ence to circular processes, lose thereby none of their authority. 



§1.1 will begin by briefly stating the principle of the equi- 

 valence of transformations, as I have already developed it, in order 

 to be able to connect with it the following considerations. 



"When a body goes through a circular process, a certain amount 

 of external work may be gained, in which case a certain quantity 

 of heat must be simultaneously expended ; or, conversely, work 

 may be expended and a corresponding quantity of heat may be 

 gained. This may be expressed by saying : — Heat can be trans- 

 formed into work, or work into heat, by a circular process. 



There may also be another effect of a circular process : heat 

 may be transferred from one body to another, by the body which 



