158 REPORT—1863. 
54, Electromotive Force of Chemical Affinity.—When two substances 
haying a tendency to combine are brought together and enter into combina- 
tion, they enter into a new state in which the intrinsic energy of the system 
is generally less than it was before, that is, the substances are less able to 
effect chemical changes, or to produce heat or mechanical action, than before, 
The energy thus lost appears during the combination as heat or electrical 
or mechanical action, and can be measured in many cases *, 
The energy given out during the combination of two substances 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 combination may be measured by the number of electrochemical 
equivalents 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, Ry 
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 
NIE 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 
WHO: 
By the definition of electrochemical equivalents, E=N, 
therefore I=E; 
or the force of chemical affinity may in these cases be measured as electro- 
motive force. 
This method of ascertaining the electromotive force due to chemical com- 
bination, which gives us a clear insight into the meaning and the measure- 
ment of ‘¢ chemical affinity,” is due to Professor W. Thomson t. 
The field of investigation presented to us by these considerations is very 
wide. We have to measure the intrinsic energy of substances as dependent 
on volume, temperature, and state of combination, When this is done, the 
energy due to any combination will be found by subtracting the energy of 
the compound from that of the components before combination, 
As the tendency to increase in volume is measured as pressure, and as the 
tendency to part with heat is measured by the temperature, so in chemical 
dynamics the tendency to combine will be properly measured by the electro- 
motive force of combination. . 
55,—Tables of Dimensions and other Constants :— 
Fundamental Units. 
Length=L, Time =T, Mass=M. 
* Report British Association, 1850, p. 63, and Phil. Mag. vol. xxxii. Ser. 3. See papers 
by Prof. Andrews, and Favre and Silbermann, ‘On the Heat given out in Chemical Action,” 
Comptes Rendus, vols. xxxvi. and xxxvu. 
+ See Rankine “ On the General Law of Transformation of Energy,” Phil, Mag. 1853. 
{ “On the Mechanical Theory of Electrolysis,” Phil, Mag, Dec. 1851. 
