746 REPORT—1885. 
smoke with only one kind of electricity in order to cause the particles. 
to combine or coagulate together—doubtless because of minute differences. 
of potential between them: the aggregations soon becoming very large 
and giving the appearance of snow. 
I now fancy that this same phenomenon of aggregation may go on 
among the atoms of a gas when it is electrified, and may account for the 
formation of ozone near an electric machine, for the formation of ammonia 
from nitrogen and hydrogen, and such like. 
(¢) Joule’s or Helmholtz’s or Thomson’s law of the dependence of 
decomposition H.M.F. on chemical combination-energy. 
The deduction of this law from the first law of thermodynamics may 
be exhibited perhaps most clearly and briefly as follows : 
(i.) Definition of E.M.F. as—the work done in a circuit per unit of 
electricity conveyed, or E= W/Q. 
(ii.) Definition of Electro-chemical Equivalent as—the mass of sub- 
stance decomposed per unit of electricity conveyed, or « = m/Q. 
(iii.) Definition of Thermal Equivalent as—the heat set free during 
the formation of one gramme of the substance from the two radicles into 
which it has just been supposed to be decomposed, provided that none of 
the energy remains in some form other than that of heat, 0 = H/m. 
(iv.) Statement of the first law of thermodynamics applied to the 
electrical decomposition of the said substance in the given way, on the 
assumption that the whole of the work done by a current is expended in 
decomposing the substance, 
Wies JH. 
The simple algebraic consequence of these four equations is 
HK = Jé6, 
which is shorthand for the law, and may be read thus:—the H.M.F. 
needed to decompose a substance into given constituents is calculable by 
simple energy considerations from purely thermo-chemical data. 
If 6” stand for the heat production per dyad gramme-equivalent of 
the substance (e.g. per 18 grammes of water, 98 grammes of sulphuric: 
acid, or 136 grammes of chloride of zinc), it is easy to see that 
ppnow dure, 
—~ 46,000 "°? 
— 6 = wee Piet gi = 2u6 hb i ] i 
for J = 42 x 108, « 96608" z, > Where p is the molecular weight 
of the substance as compared with an atom of hydrogen, and & is the 
atomicity or number of bonds loosed in the decomposition supposed. 
Values of 6” are tabulated direct by Julius Thomsen for a great 
variety of substances, and are quoted in Naumann’s ‘ Chemie,’ vol. i. and 
also partly in Watt’s ‘ Dictionary of Chemistry’; hence the obtaining of 
the volts needed to decompose a substance, by simply dividing 6” by 46,000, 
is extremely convenient. 
