CHAP, xi.] ELECTRICITY AND MAGNETISM. 381 



sition of Q^ grammes of an ion whose absolute electro- 

 chemical equivalent is z. [For example, z for hydrogen . 

 is -000105 gramme, being ten times the amount (see 

 table in Art. 212) deposited by one weber, for the weber 

 is iV of the absolute C.G.S. unit of quantity.] If H 

 represent the number of heat units evolved by one 

 gramme of the substance, when it enters into the com- 

 bination in question, then Q^H represents the value (in 

 heat units) of the chemical work done by the flow of the 

 Q units ; and this value can immediately be translated 

 into ergs of work by multiplying by Joule's equivalent J 

 (= 42 x io 6 ). 



We have therefore the following equality : 

 EQ = Q^HJ ; whence it follows that 



E = HJ ; or, in words, the electromotive- 

 force of any chemical reaction is equal to the product of 

 the electro-chemical equivalent of the separated ion into 

 its heat of combination, expressed in dynamical itnits. 



EXAMPLES. (i) Electromotive -force of Hydrogen tend- 

 ing to unite with Oxygen. For Hydrogen, z = 

 000105 ; H (heat of combination of one gramme) 

 = 34000 gramme-degree-units ; J = 42 x io 6 . 



000105 "x" 34,000 x 42 x 10 = 1-45 x io 8 

 "absolute" units of electromotive-force, or = 1*45 

 volts. 



(2) Electromotive -force of Zinc dissolving into Sulphuric 



Acid. 0= -003412 ; H=i6;o (according to Julius 

 Thomsen) ; J = 42 x io 6 . 



003412 x 1670 x 42 x io 6 = 2 -394 x io 8 . 

 or = 2*394 volts. 



(3) Electromotive-force <?/" Copper dissolving into Sulphuric 



Acid. z - -003307 ; H = 88 1 ; J = 42 x io 6 . 



003307 x 881. x 42 x io 6 = 1-223 x io 8 . 

 or = 1.223 volts. 



(4) Electromotive -force of a Daniell's Cell. Here zinc is 



dissolved at one pole to form zinc sulphate, the 

 chemical action setting up a + electromotive-force, 



