240 Dr. C. R. A. Wright on the Determination of 



cle Chim. et de Phys. 4th series, ii. p. 317, and iv. p. 392) by a 

 method which will be discussed later on (§ 23). The amount 

 of heat absorbed per gramme-equivalent of water decomposed 

 was found by Raoult to be 34028, or sensibly the same as that 

 deduced by direct combustion ; whilst similar results were 

 obtained with copper sulphate. In 1869 Kiechl published 

 experiments (Wien. Ahad. Ber. lx. [2] 121) in principle the 

 same as Joule's, but differing considerably in the details, no- 

 tably in that the wire was made into a rheostat, so that its 

 resistance could be regulated throughout the observations. 

 As a mean of nine accordant experiments, 33653 was found to 

 be the heat absorbed per gramme-equivalent of water decom- 

 posed. Shortly before this, Favre demonstrated in a very 

 simple way (Cornptes Rendus, lxvi. p. 252 ; Pogg. Ann. cxxxv. 

 p. 300) that as much heat is absorbed during the decomposi- 

 tion of an electrolyte as is produced by the recombination of 

 the products of decomposition. A small battery was enclosed 

 in a calorimeter, and the heat produced by the solution of a 

 given quantity of zinc determined, the current produced being 

 made to circulate through a coil of wire also contained in the 

 calorimeter; so that the total heat generated was determined. 

 A voltameter was then substituted for the coil of wire, when 

 the quantity of heat was found to be lessened by an amount 

 equal to 34,204 gramme-degrees per gramme-equivalent of 

 zinc dissolved, i. e. of water decomposed. Similarly when 

 copper sulphate was decomposed instead of water, the heat- 

 evolution was lessened by an amount sensibly equal to the 

 heat evolved in the synthesis of the salt. 



6. In 1851 Sir William Thomson carried the ideas suggested 

 by the experiments of Joule and others a considerable way 

 further. In a remarkable paper on the mechanical theory of 

 electrolysis (Phil. Mag. 1851, vol. ii. p. 429), he demonstrated 

 that the force of chemical affinity is expressible in terms of 

 electromotive force, and calculated, from data largely fur- 

 nished by Joule, that the work done in decomposing water 

 corresponds to an E.M.F. of 3,305,400 foot-grain-second 

 electromagnetic units (equal to 1*415 x 10 8 C.G.S. units, or 

 1'415 volt), that representing the sum of the chemical actions 

 in a Daniell cell corresponding to 2,507,100 (equal to 1*074 

 x 10 8 C.G.S. units); so that the "affinity" between oxygen 



1*415 

 and hydrogen in water is =1*318 times as strong as the 



sum of the affinities exerted in the chemical changes going on 

 in a Daniell cell. Similar calculations, based on more recent 

 determinations of the values involved, lead to somewhat higher 

 numbers, approximating to 1*502 x 10 8 C.G.S. units for water 



