406 Prof. R. Clausius on the Second Fundamental 



heat a connexion of such a kind that mechanical work can be ob- 

 tained by the expenditure of heat, and, conversely, heat can be 

 produced by the expenditure of work, there being under all cir- 

 cumstances one constant ratio between the quantity of heat and 

 the amount of work. This principle, known as the Principle of 

 the Equivalence of Heat and Work, formed the starting-point 

 of the rapid development which the mechanical theory of heat has 

 undergone in recent times. 



In connexion with this principle, I may be allowed to make 

 a remark which may contribute to facilitate the exposition of 

 what follows. 



When work is produced with accompanying expenditure of 

 heat, or heat is produced while work is expended, this may be 

 briefly expressed by saying that heat is transformed into work, or 

 work into heat. Two such magnitudes, capable of being con- 

 verted one into the other, so that one of them may serve to replace 

 the other, will naturally often require to be considered together ; 

 in a mathematical sense, they must be regarded as of the same 

 kind, and occasions will often arise when it is needful to add 

 them together or subtract them one from the other. In such 

 cases considerable inconvenience arises from the circumstance 

 that heat and work are measured by reference to different stand- 

 ards. The unit of work is taken, as is well known, as the pro- 

 duct of unit- weight into unit-length, or, for example, on the 

 French metrical system as one kiloguammetre ; whereas we are 

 accustomed to take as the unit of heat the quantity of heat which 

 is required to raise the temperature of unit-weight of water from 

 0° to 1° C. While employing these units, we cannot at once 

 speak of the sum of heat and work; in order to make the sum 

 we must either reduce the work to heat-units, or the heat to 

 work-units. When this is done we always obtain complicated 

 expressions such as these, — "the sum of the heat and the ther- 

 mal equivalent of the work," or " the sum of the work and the 

 mechanical equivalent of the heat." 



On this account I have proposed to introduce, as well as work, 

 a second magnitude, which, while still representing work, ex- 

 presses it in terms, not of the above-mentioned mechanical unit, 

 but in terms of the unit of heat, and therefore so that that 

 amount of work is taken as the unit of work which is equivalent 

 to the thermal unit. For work, when thus measured, I have pro- 

 posed the name Ergon*. The principle that heat can be con- 

 verted into ergon and ergon into heat, then, still holds good just 

 as for work, and we have at the same time the simple relation 



* [In German WerJc. See foot-note to page 253 of the author's 'Me- 

 chanical Theory of Heat/ English edition by Hirst (Van Voorst, 1867). — 

 Transl.] 



