GENERAL PRINCIPLES 43 



pere.-f 1 ) and Fresnel.(*p he latter natural philosopher declared 

 in very clear terms that heat is a vibration. And until that date 

 (1822) the relation between xvoi'k and heat in such transformations 

 was only considered to Be qualitative. The possibility of a 

 quantitative' relation was not then foreseen. 



In 1824 Sadi-Carnot ( 3 ) investigated the production of work 

 at the expense of heat in the steam engine. Comparing thermal 

 engines with hydraulic engines, he recognised that the power 

 of a steam engine depends on the difference of temperature 

 between the boiler and the condenser, in the same way that the 

 power of a water wheel depends on the difference of the level of 

 the mill-race above and below. In some posthumous notes, only 

 published in 1878 (forty-six years after his death) Carnot ad- 

 mitted a destruction of heat in its transformation into work. 

 He had therefore arrived at the law of the " equivalence " of heat 

 and work. But his thought was specially arrested by the fact 

 that without a fall of temperature, without a rupture of thermal 

 equilibrium there was no possibility of motive power being pro- 

 duced. The maximum power, in other words, the greatest 

 quantity of work of a thermal engine, is fixed by that difference 

 of temperature. Already Pictet ( 4 ) had attributed a " tension " 

 to heat and Berthollet wrote these curious lines : " We can com- 

 pare this tension to the effort of an elastic substance which puts 

 itself in equilibrium of elasticity with other similar substances 

 which react upon it ; its effect is all the greater the more differ- 

 ence there is between their temperatures ; from which we can 

 draw the conclusion that the caloric acts with more energy on 

 bodies of which the temperature is different, and the tension is 

 greater. ( 5 )" This principle, affirmed by Berthollet in 1803, 

 appears to us to have inspired all the authors, after Carnot. 

 " We can say," taught Poncelet in 1826, " that a certain 

 quantity of heat must develop, against the resistances directly 

 opposed to its action, quantities of absolute work, which are 

 always the same or independent of the method of that action 

 and the nature of the bodies. This principle is somewhat 

 analogous to that put forward by M. Sadi-Carnot, an old 

 pupil of the Ecole Polytechnique, in a little work called 

 Reflexions. . . . ( 6 ) " 



(*) Ampere Ann. phys. et Chimie, 1821, vol. LVIII, p. 432. 



( 2 ) Fresnel, De la Lumiere : addition to the Chemistry of Thompson, 

 1822. 



( 3 ) S. -Carnot, Reflexions sur la Puissance Motrice du Feu (1824) : new 

 editions 1887 and 1903 (Hermann). 



( 4 ) Pictet, Essais de Physique. 



( 5 ) Berthollet, Essai de Statique Chimique, vol. I. p. 155; (1803). 



( 6 ) Poncelet, Mecanique Industrielle, p. 216 (note). 



