John Prescott Joule 27 



generating heat by mechanical force in different manners, 

 but always finding the same equivalent, until no vestige of 

 doubt was left that all different forms of energy could be 

 expressed in the same units. His measurements became 

 more and more accurate, and such uncertainties as remained 

 in the numerical value of the equivalent were, in great part, 

 due to the difficulty of measuring the temperature with a 

 glass thermometer ; the accuracy obtained was indeed to 

 some extent the result of the accidental excellence of his 

 thermometers. A few years later the composition of glass 

 became much less suitable for scientific use. 



It has already been noted that while the conversion 

 of mechanical work into heat was completely and satis- 

 factorily dealt with by Joule, the converse transformation 

 of heat into work involves further important considerations, 

 into which it was necessary to enter. Sadi Carnot had, in 

 1824, published a work entitled " Reflexions sur la puis- 

 sance motrice du feu, et sur les machines propres a developper 

 cette puissance," in which the subject was treated with 

 masterly perspicuity, but his reasoning was expressed in 

 the language of the material theory of heat. He was, however, 

 the first to point out that the mechanical production of 

 an effect by a heat engine is always accompanied by a 

 transference of heat from one body to another at a lower 

 temperature. Relying on the axiom that a perpetual motion 

 involving a continuous performance of work is impossible, 

 he laid down the conditions for a thermodynamic engine 

 which, with a given transference of heat, would do the 

 maximum amount of work. The peculiarity of such an 

 engine is, that whatever amount of work can be derived 

 from a certain transference of heat, an equal reverse thermal 

 effect will be produced if the same amount of work be spent 

 in working it backwards. Further, the work done by a 

 perfect heat-engine must be the same for the same trans- 

 ference of heat, whatever be the nature of the material 

 used. If heat be a form of energy, and not a substance, 

 it is clear that the amount which enters the cooler body 

 of an engine must be less than that which leaves the 

 hotter one, and that the difference is equivalent to the 

 mechanical work done in the passage. The position of 



