Relations between Electrical Measurements. 521 



The energy thus lost appears during the combination as heat 

 or electrical or mechanical action, and can in many cases be 

 measured*. 



The energy given out during the combination of two sub- 

 stances may, like all other forms of energy, be considered as the 

 product of two factors t — the tendency to combine, and the 

 amount of combination effected. Now the amount of combina- 

 tion may be measured by the number of electrochemical equiva- 

 lents which enter into combination ; so that the tendency to 

 combine may also be ascertained by dividing the energy given 

 out by the number of electrochemical equivalents which enter 

 into combination. 



If the whole energy appears in the form of electric currents, 

 the energy of the current is measured by the product of the 

 electromotive force and the quantity of electricity which passes. 

 Now the quantity of electricity which passes is equal to the 

 number of electrochemical equivalents which enter on either side 

 into combination. Hence the total energy given out, divided 

 by this number, will give the electromotive force of combination. 

 Thus, if N electrochemical equivalents enter into combination 

 under a chemical affinity I, and in doing so give out energy 

 equal to W, either as heat or as electrical action, then 



NI=W. 



But if W be given out as electrical action, and causes a quantity 

 of electricity Q to traverse a conductor under an electromotive 

 force E, we shall have 



W=EQ. 



By the definition of electrochemical equivalents, 



E=N, 

 therefore 



I=E; 



or the force of chemical affinity may in these cases be measured 

 as electromotive force. 



This method of ascertaining the electromotive force due to che- 

 mical combination, which gives us a clear insight into the mean- 

 ing and the measurement of " chemical affinity," is due to Pro- 

 fessor W. Thomson {. 



The field of investigation presented to us by these considera- 



* Report of the British Association, 1850, p. 63 ; and Phil. Mag. 

 S. 3. vol. xxxii. See papers by Professor Andrews, and Favre and Silber- 

 mann, " On the Heat given out in Chemical Action/' Comptes Rendus, 

 vols, xxxvi. and xxxvii. 



t See Rankine " On the General Law of Transformation of Energy," 

 Phil. Mag. 1853. 



X " On the Mechanical Theory of Electrolysis," Phil. Mag. Dec. 1851. 

 Phil Mag. S. 4. No. 199. Suppl. Vol. 29. 2 M 



