Chemical Affinity in term* of Electromotive Force. 241 



and 1*154 x 10 8 for the Daniell cell (vide § 16), giving the 



ratio ™= 1*302. 

 1*154 



7. The general proposition that the force of chemical affi- 

 nity is expressible in terms of E.M.F. may be simply demon- 

 strated as follows: — Let the strength of chemical combination, 

 or the chemical work done by the force of chemical affinity, be 

 defined as the absolute amount of work spent or gained in 

 the synthesis from its constituents of a given weight (say 

 1 gramme) of the compound considered. This work may be 

 expressed in terms of heat, and is ordinarily measured as such : 

 thus in the synthesis of a gramme of water from gaseous 

 oxygen and hydrogen at the ordinary pressure and tempe- 

 rature, an amount pf work is done representing an evolution 

 of a quantity of heat amounting, according to the most accu- 

 rate determinations, to close upon 3800 gramme-degrees 

 (§ 31); whence, taking the mechanical equivalent of heat as 

 42 megalergs per gramme-degree*, the work done in this 

 synthesis is close to 16 erg-tens =16 x 10 10 C.Gr.S. units. 



The experiments of Faraday first demonstrated the two fol- 

 lowing laws, which have since been fully confirmed by many 

 other experimenters: — 



I. When a compound is electrolyzed, the weight of sub- 

 stance decomposed is proportionate to the quantity of electri- 

 city passing. [A low degree of conductivity is supposed by 

 some experimenters to be possessed by electrolytes, of such a 

 nature that very feeble currents can be passed through them 

 without causing any chemical decomposition ; this, however, 

 even if admitted to be the case, would not appreciably inter- 

 fere with the correctness of the foregoing proposition.] 



II. When a current is passed for a given time successively 

 through several compounds so as to electrolyze them all, it 

 decomposes quantities of them in the ratio of simple functions 

 of their " molecular weights," evolving the products of decom- 

 position, when elementary, in the ratio of simple functions of 

 their combining numbers or "atomic weights." The quanti- 

 ties of compounds decomposed and of elements evolved by a 

 current which evolves 1 part of hydrogen from water under 

 these circumstances are called their " equivalent values," or, 

 more briefly, their equivalents, in reference to the particular 

 compounds decomposed. 



* The values obtained by Joule by friction of water lie somewhat 

 below this figure (near to 41'5xl0 3 ); whilst that deduced by him from 

 the development of heat by electric currents in a wire of known resistance 

 lies somewhat above it (42-10 X 10 6 ). Numbers lying- somewhat above 

 42xl0 6 have been obtained by Hirn, Violle, and Regnault (vide §§ 33 & 

 34). 



