49 



The recent discoveries* of the generation ol" heat ili rough the 

 friction of fluids in motion, and by the magneto-electric excitation 

 of galvanic currents would, either of them, be sufficient to demon- 

 strate the immateriality of heat, and would so aff'ord, if required, a 

 perfect confirmation of Sir Humphrey Davy's views. 



Although Sir Humphrey Davy had established beyond all doubt 

 the fact that heat may be created by mechanical work, the converse 

 proposition, that heat is lost when mechanical work is produced from 

 thermal agency, appears to have been first enunciated by Mayer in 

 1841. In 1842 the same proposition was enunciated by Joule, and 

 a number of most admirable experiments illustrating the mutual 

 convertibility of heat and mechanical effect, and the constancy of 

 thermal effects through the most varied means, from given causes, 

 are described in his paper on Magneto-electricity, and adduced in it 

 from his former experimental researches by which the laws of the evo- 

 lution of heat by the galvanic battery had been established. The 

 same paper contains the first investigation on true principles that has 

 ever been made of the numerical relations which conne-jt heat and 

 mechanical effect ; and numerical determinations of " the mechanical 

 equivalent of a thermal unit" are given as the results of two 

 classes of experiments, in each of which mechanical work is spent, 

 and no other final effect than the creation of heat is produced, in 

 one class by means of magneto-electric currents, and in the other, by 

 means of the friction of fluids in motion. 



In subsequent experimental researches he has made more ac- 

 curate determinations, and, from his last set of experiments on the 

 friction of fluids, he concludes " that the quantity of heat capable 

 of raising the temperature of a pound of water (weighed in vacui' 

 and taken at between 55' and 60°) by 1° Fahr., requires for its 

 evolution the expenditure of a mechanical force represented by the 

 fall of 772 lb. through the space of one foot." 



* In May 1842, Mayer announced, in the Annalen of Wohler and Liebig, 

 that he had raised the temperature of water from 12° to 13° cent., by agitoting 

 it. In 1843, Joule announced in the Philosophical Magazine that •• heat is 

 evolved by the passage of water through narrow tuboj* ;'" and in the month of 

 August of that year (1843), he announced to the British Associntion that heat 

 is generated when work is spent in turning a inagueto-eloctric machine, or an 

 electro-magnetic engine. (Sec his paper "on the Calorific KfTccts of Magnatn- 

 Blecfcricity and on the Mechanical Value of Heat." Phil. Map vol. x.xiii. 1843.) 



