ON ELECTROLYSIS. 761 
s0 a = 4:5 x 1019, 23, 
and yf = 767" 108 et 
The most probable value of « at the present time is, I suppose, + 10-8 ; 
and the fact that 7 comes out so nearly equal to z is at least noticeable. 
Distinction between electrolytes and dielectrics. 
Homogeneous electrolytes and dielectrics behave very similarly, when 
supplied with electrodes at slight difference of potential, but the distinc- 
tion is that, whereas in an electrolyte the whole of the strain is thrown 
upon a pair of thin films, the bulk of the medium being as quiescent and 
unstrained as a metal, in a dielectric the strain exists throughout the 
medium, sloping steadily down from anode to cathode; if these terms be 
still permissible. In the one therefore the _ strain is excessive, and 
very small differences of potential easily cause decomposition ; in the other, 
the strain on each molecule is insignificant—there being a whole series of 
them to share it—and accordingly it takes a great difference of potential 
to effect decomposition, or disruptive discharge as it isin this case called, 
especially if mechanical locomotion of the medium, or a chain of dust 
particles, be avoided. 
In rare gases it seems likely that, by a locomotion of molecules, a steady 
convective discharge can be maintained, distinct from true disruption; 
indeed mobility of particles may play an important part in the giving 
way of any fluid. But, ignoring this, we can reckon roughly on electro- 
static principles how much H.M.F. a given thickness of a dielectric ought 
to bear; and this is what, at the beginning of the present section, I call 
(6) Calculation of E.M.F. needed to decompose a dielectric. 
Let the distance of the plates be z; there will be a series of ~ mole- 
x 
cules to share the V—V’ between them. 
Now, since there are about 10° molecules per linear centimetre in. a 
liquid, and about 10° in an ordinary gas, the slope of potential in a die- 
lectric will be some 10-*th of what it is in the films of an electrolyte for 
the same potential difference ; and accordingly, since it takes one or two 
volts to decompose a liquid, it will require one or two hundred million volts 
per centimetre to burst a liquid dielectric, and a tenth of this for a gas. 
This is probably much too high—certainly it is for a gas; but then what 
is one to think about the electrification of the atoms of a simple gas like 
nitrogen? The hint of Helmholtz occurs to one, that possibly even mole- 
cules of elements possess some electric charges; and if one could fancy 
that the atomic charge in such molecules, instead of being 10-", was 
something like 10-", the experimental value of the dielectric strength of 
air (viz. 33,000 volts per centimetre) would be obtained, on the preceding 
hypothesis ; and it would vary with the cube root of the pressure.! 
_ | On any such theory, the bursting potential will vary as the number of molecules 
in a row between the plates, i.e. as the cube-root of the pressure in the case of a 
gas. This is by no means true for the potential required to begin discharge, but it 
is not hopelessly out of accord with some measurements by Réntgen (Wied. Elek. 
av. 465) of the minimum potential sufficient to maintain a discharge already begun 
