ACCORDANCE OF QUANTITATIVE THEORIES, &c. 211 



must be given, or received, in a definite change of tem- 

 perature effected in a definite quantity of a standard sub- 

 stance, such as water. No less than seven almost entirely 

 distinct modes of determining this constant have been 

 tried. Dr. Joule first ascertained by the friction of water 

 that to raise the temperature of one kilogram of water 

 through one degree centigrade, we must employ energy 

 sufficient to raise 424 kilograms through the height of one 

 metre against the force of gravity at the earth's surface. 

 Joule, Mayer, Clausius h , Favre and other experimentalists 

 have made various other determinations by less direct 

 methods, and their results may be thus summed up 1 . 



Friction . -f 



(413 



Mechanical properties of gases . . 426 



Work done by a steam engine . . 413 



Heat evolved by induced electric currents 452 



Heat evolved by electro-magnetic engine 443 



Heat evolved in the circuit of a battery 420 



Heat evolved by an electric current . 40x3 



Considering the diverse and in many cases difficult 

 methods of observation, these results exhibit satisfactory 

 accordance, and their mean (423*9) comes very close to 

 the number derived by Dr. Joule from the apparently 

 most accurate method. The constant generally assumed 

 as the most probable result is 423*55 kilogrammetres, 

 or gramme metres, if the quantity of water heated i Cent. 

 be one gramme instead of a kilogramme. 



h Clausius, 'Philosophical Magazine/ 4th Series, vol. ii. p. 119. 

 i Watts' 'Dictionary of Chemistry/ vol. iii. p. 129. 



P 2 



