184: ANNUAL OF SCIENTIFIC DISCOVERY. 



the heat developed by the friction is not produced from the solids, but is called 

 into existence between them. An unfortunate use of the word " capacity for 

 heat," which has been the occasion of much confusion ever since the discovery 

 of latent heat, and has frequently obstructed the natural course of reasoning on 

 thermal and thermo-dynamic phenomena, appears to have led both Rurnford 

 and Davy to give reasoning which no one could for a moment feel to be con- 

 clusive, and to have prevented each from giving a demonstration which would 

 have established once and for ever the immateriality of heat. 



Another case of apparent loss of work, well known to an audience in the 

 Royal Institution that in which a mass of copper is compelled to move in the 

 neighborhood of a magnet was adduced ; and an experiment was made to 

 demonstrate that in it also heat appears as an effect of the work which has 

 been spent. A copper ball, about an inch in diameter, was forced to rotate 

 rapidly between the poles of a powerful electro-magnet. After about a 

 minute it was found by the thermometer to have risen by 15 Fahr. After 

 the rotation was continued for a few minutes more, and again stopped, the ball 

 was found to be so hot that a piece of phosphorus applied to any part of its 

 surface immediately took fire. It is clear that in this experiment the electric 

 currents discovered by Faraday to be induced in the copper by virtue of its 

 motion in the neighborhood of the magnet, generated the heat, which became 

 sensible. Joule first raised the question, Is any heat generated by an 

 induced electric current in the locality of the inductive action? He not only 

 made experiments which established an affirmative answer to that question, 

 but he used the mode of generating heat by mechanical work established by 

 those experiments, as a way of finding the numerical relation between units 

 of heat and units of work, and so first arrived at a determination of the 

 mechanical value of heat. At the same time (1843) he gave another determina- 

 tion founded on the friction of fluids in nlotion ; and six years later he gave 

 the best determination yet obtained, according to which, it appears that 772 

 foot pounds of work (that is, 772 times the amount of work required to over- 

 come a force equal to the weight of 1 Ib. through a space of one foot) is 

 required to generate as much heat as will raise the temperature of a pound of 

 water by one degree. 



The reverse transformation of heat into mechanical work was next consi- 

 dered, and the working of a steam engine was referred to as an illustration. 

 An original model of Stirling's air engine was shown in operation, developing 

 motive power from heat supplied to it by a spirit lamp, by means of the alter- 

 nate contractions and expansions of one mass of air. Thermo-electric currents, 

 and common mechanical action produced by them, were referred to as illustrat- 

 ing another very distinct class of means by which the same transformation may 

 be effected. It was pointed out that in each case, while heat is taken in by 

 the material arrangement or machine, from the source of heat, heat is always 

 given out in another locality, which is at a lower temperature than the locality 

 at which heat is taken in. But it was remarked that the quantity of heat 

 given out is not (as Carnot pointed out it would be if heat were a substance) the 

 same as the quantity of heat taken in, but, as Joule insisted, less than the 

 quantity taken in by an amount mechanically equivalent to the motive 



